File:Analysis of development (1955) (17547692494).jpg

Title: Analysis of development
Identifier: analysisofdevelo00will (find matches)
Year: 1955 (1950s)
Authors: Willier, Benjamin H. (Benjamin Harrison), b. 1890
Subjects: Embryology; Embryology
Publisher: Philadelphia, Saunders
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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324 Embryogenesis: Progressive Differentiation pared to the pre-trochal, and the larger to the post-trochal region, the large vacuoles occupy approximately the position of the prototroch. It is interesting to note that a similar girdle of large vacuoles is found in this position in the trochophore. Continuing the comparison, we may note that the aggre- gation of yolk is in a similar position to the gut of the trochophore." Unusually strong and active cilia were formed and were ar- ranged with great regularity. Thus consid- erable differentiation can occur when cleav- age fails to segregate sharply localized cyto- plasmic regions.
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Fig. 122. Differentiation without cleavage in the annelid Chaetopterus (from Lillie, '02). A, Experi- mental larva; optimal differentiation without cleav- age. B, Control larva. ctenophores and tunicates, it is essential that the chemical and physical nature of these areas be analyzed by cytochemical methods or by any other methods which can be ap- plied to minute objects. Examples of this approach to the study of differentiation have been illustrated, summarized and evaluated by Needham ('42, pp. 131-140) and by Brachet ('50, pp. 271-291). Needham states (p. 139), ". . . in many eggs showing mo- saic development a profound chemical het- erogeneity of the parts of the egg sets in very early, parallel with the early deter- mination. Histochemical and experimental methods confirm each other. The former alone will not carry us much further, but with the micro-chemical technique which should before long be available, it ought to be possible to establish by unimpeachable chemical methods the differences between the determined areas." A step in this direc- tion has been taken by Berg and Kutsky ('51), who studied differences in oxygen uptake of isolated blastomeres and polar lobes in the egg of the moUusk Mytilus edulis. They report a lower respiratory rate of the cytoplasm of the CD blastomere when compared with the AB blastomere and dem- onstrate that the lower rate of the former is due to the lower respiratory rate of the polar lobe cytoplasm which is incorporated into the CD cell. The importance of the polar lobe cytoplasm in development has been stressed above. Although similar results have been ob- tained by other investigators, this still re- mains the classic example of this phe- nomenon. This experiment would seem to indicate the primary importance in differ- entiation of the precocious localization of different cytoplasmic areas and the secondary importance of the segregation of these areas by the cleavage process.* CHEMICAL CHARACTERIZATION OF LOCALIZED CYTOPLASMIC AREAS IN MOSAIC EGGS If precocious localization of cytoplasmic areas is of primary importance for the dif- ferentiation of eggs of annelids, moUusks, * Brachet ('37) has demonstrated that such dif- ferentiation without cleavage is impossible when oxidations are inhibited by cyanide, and also when metabolism is interfered with by monoiodoacetic acid, although some localization of cytoplasmic ma- terials can take place in the presence of the latter agent. INTERACTION BETWEEN BLASTOMERES IN DEVELOPMENT IN MOSAIC EGGS In the preceding section emphasis was placed on the independent differentiation of blastomeres in certain phyla, i.e., on the apparent lack of interaction between blasto- meres. Thus far interpretations have been based largely upon results of defect ex- periments or isolation experiments. Atten- tion may now be turned briefly to experi- ments involving transplantation, fusion of eggs, etc., to see whether they provide any evidence for interaction between blasto- meres in nemertines, annelids, moUusks or tunicates. Horstadius ('37a) found that from the 8-cell stage onward, separation of the egg of the nemertine Cerebratulus into ani- mal and vegetal halves, into a/Zj, aA22, veg-^ and veg2 layers, various combinations of these layers of blastomeres, as well as fu- sion of animal halves with meridional halves, provided no evidence for interaction

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