Cleavage, Blastulation and Gastrulation 



217 



parthenogenetically, but with somewhat 

 dubious success. McClendon reported that 

 cells could segment irregularly without nu- 

 clei but it is not certain that his Asterias 

 egg-fragments were completely devoid of 

 chromatin. Fry appears to have refuted the 

 earlier evidence of Wilson ('01a,b) that as- 

 ters can divide in the absence of chromatin. 

 Fry obtained an irregular single cleavage in 

 some of these fragments. 



Fankhauser ('29, '34) describes amphi- 

 asters and spindles devoid of chromosomes in 

 merogonic Triton embryos; these appeared 

 to cause division of non-nucleated cells. In 

 these same embryos, however, nucleated cells 

 were also present, and were considered to 

 be essential for the continued life of the 

 chromatin-free cells. 



Gross ('36), after cold-treating embryos 

 of Artemia salina, obtained cleavage of cer- 

 tain blastomeres devoid of nuclei. These divi- 

 sions apparently occurred under the influence 

 of the cytasters which were observed in the 

 non-nucleated cells. 



The general problem was again brought 

 into focus by the observations of E. B. Har- 

 vey on the eggs of Arbacia ('36), of other 

 species of sea urchins ('38, '40), and of 

 Chaetopterus ('39). Non-nucleated fragments 

 of Arbacia eggs obtained by centrifuging 

 were treated with parthenogenetic agents. 

 Cytasters appeared but no spindles, and four 

 to six hours after treatment, a "cleavage 

 plane" sometimes appeared between two 

 cytasters. Cleavage is reported to have con- 

 tinued in certain of these eggs, resulting in 

 the formation of blastulae, which could 

 emerge from the fertilization membrane. 

 There is no good evidence that these "em- 

 bryos" ever differentiated cilia. Chaetopterus 

 "embryos" did not develop this far. Two 

 qviestions. at least, that might be raised are 

 (1) whether this is really cleavage, as de- 

 fined above, or merely a type of cellular 

 fragmentation, and (2) whether the fvirrow- 

 ing is brought about by the cytasters. Ade- 

 quate evidence concerning these points is 

 not available from the work on the sea ur- 

 chin egg. 



If one should be willing to concede 

 that "parthenogenetic merogony" is a form 

 of cleavage, the significance of the observa- 

 tions is still open to question. Cleavage of 

 non-nucleated fragments of the sea urchin 

 egg is not cleavage without nuclear material. 

 The sea urchin egg, as obtained from the 

 mature female, is a fully mature egg, with 

 female pronucleus present, both maturation 

 divisions having been completed. When, in 



the ovary, the oocyte nucleus (germinal 

 vesicle) ruptured during the formation of 

 the first maturation spindle, all the residual 

 substance of the large germinal vesicle was 

 set free in the cytoplasm. This nuclear ma- 

 terial has long been considered to cendition 

 the egg cytoplasm for activation and cleav- 

 age (O. and R. Hertwig, '87; Delage, '99; 



Fig. 71. Strongly centrifuged (61 minutes at 

 66,000 times gravity) Nereis egg showing zones of 

 stratification, in order from centripetal pole, as 

 follows: large oil drops, hyaline protoplasmic layer, 

 an indistinct layer of fine neutral red-staining gran- 

 ules, zone of yolk spheres, a second hyaline zone, 

 and a centrifugal zone of small jelly-precursor gran- 

 ules, containing a vortex of heavy neutral red-stain- 

 ing granules. (From Costello, '40.) 



Wilson, '03; Yatsu, '05; Lillie, '19; and 

 others). A real test of cleavage without 

 nuclear material could therefore be obtained 

 only on fragments of eggs taken before 

 germinal vesicle breakdown. Such an in- 

 vestigation, requiring an egg fertilizable in 

 the germinal vesicle stage, was reported by 

 Costello ('40). Eggs of Nereis limbata, which 

 had been stretched by prolonged centrifug- 

 ing at about 66,000 g., were cut into two 

 halves within the vitelline membrane. The 

 fragments lacking the germinal vesicle could 



