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. 



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 



