298 



Embryogenesis: Progressive Differentiation 



poration of marked material from the yolk 

 by the blastodisc. 



In the early days of morphogenetic experi- 

 mentation (Morgan, 1893; also Lewis, '12, 

 Hoadley, '28) it was established that con- 

 siderable cytoplasm of the uncleaved blasto- 

 derm in Fundulus may be removed by punc- 

 ture without disturbing subsequent develop- 

 ment. More recently (Tung and Tung, '44), 

 there has been a report of puncturing the 

 goldfish egg in the equatorial region, just 

 before fertilization while the protoplasm 



*f-:^^- 



:..^r^ 



Fig. 108. From top to bottom: a giant embryo of 

 Carassius produced by fusion of two four-celled 

 eggs; a normal embryo of corresponding stage; and 

 an embryo formed from half an egg (cf. Fig. 110). 

 (Redrawn from Tung and Tung, '44, Fig. 16.) 



is still flowing upward to form the blasto- 

 disc. A normal embryo may form from as 

 little as one-half the material of the egg 

 under these circumstances. All fragments, 

 however, do not form normal or nearly nor- 

 mal embryos: over half form monstrosities 

 where head, trunk or tail predominates; in 

 a few, gastrulation is suppressed. Clearly, 

 some sort of spatially arranged system is 

 being displaced in these cases. 



Polar arrangement in the vast fluid ovar- 

 ian egg of the bird is even more impressive 

 than it is in the case of the fish. The eccen- 

 tricity of the nucleus has an obvious intimate 

 relation to the pattern of yolk deposition. It 

 must be recalled, when discussing axiation in 

 the meroblastic eggs, that as yolk content 

 becomes more extreme in proportion to pro- 

 toplasm, the animal-vegetal axis tends more 

 and more to represent the future dorso- 

 ventral axis of the embryo, while the antero- 

 posterior axis of the embryo proper is repre- 



sented by a smaller and smaller meridional 

 arc of the egg. In the pigeon, Bartelmez 

 ('12) has traced not only anteroposterior but 

 left-right axes back to slight eccentricities 

 of the nucleus in early oocyte stages. In the 

 chick, too, the statistically regular orienta- 

 tion of the egg rotating in the oviduct, dur- 

 ing or immediately following fertilization, 

 would indicate previous establishment of the 

 major axes. 



THE CLEAVAGE PERIOD 



It has long been known (Clapp, 1891; Mor- 

 gan, 1893) that cleavage in the teleost egg, 

 although following a predictable pattern, 

 bears only an imprecise relation to the fu- 

 ture embryonic axis. The vital stain studies 

 of Oppenheimer ('36b) have demonstrated 

 explicitly the variability of this relation in 

 Fimdulus. These considerations have un- 

 doubtedly promoted the tendency among em- 

 bryologists to regard the cleavage period 

 as a purely mitotic interlude, not involving 

 profound changes in quantity or location of 

 critical protoplasmic materials. The figures 

 of Oppenheimer in the publication cited do 

 not indicate spatial rearrangements of any 

 magnitude within the blastodisc, until the 

 blastula stage is reached. However, addition 

 of some material to the blastodisc from the 

 surrounding yolk or periblast has been re- 

 peatedly reported (cf. Roosen-Runge, '38). 

 There is experimental evidence to indicate 

 that some progressive change takes place in 

 blastodisc, yolk, or both while the segmen- 

 tation process continues at top speed. 



Data on regulation of defects in Fundulus 

 eggs have been most recently summarized 

 by Nicholas and Oppenheimer ('42). Two- 

 and 4-celled stages may regulate 50 per cent 

 loss completely: i.e., the removal of one or 

 two blastomeres. Tung and Tung, as Figure 

 108 demonstrates, have shown for the gold- 

 fish that at this early stage not only can a 

 half egg form a complete embryo (see Fig. 

 110) but two eggs may be, imder proper con- 

 ditions, fused into one. 



In Fundiilus it appears that in the 8-celled 

 stage (Hoadley, '28) or 16-celled stage, cer- 

 tain defects, at least, can entirely prevent 

 gastrulation. The basis of this relative lack 

 of regulability in later cleavage stages is 

 not at all clear. It may be that surface rela- 

 tions of cells do not permit adequate cover- 

 ing of the wound, and that this apparent 

 progressive differentiation is not due to any 

 profound change in organization. On the 

 other hand, the 16-celled stage marks the 



