. Analysis of development. Embryology; Embryology. 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 ; Unusually strong and active cilia were formed and were ar- ranged with great regularity. Thus consid- erable differen
. Analysis of development. Embryology; Embryology. 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 ; 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 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 ; 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 C
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