DEVELOPMENT OF A CHIM.ERO1D. 2/9 



mass of bright red gill-filaments. The embryo itself next ap- 

 peared, as shown here in outline in Fig. 6. It was taken to the 

 laboratory and kept living for half a day ; and possibly, like 

 kindred shark embryos, it would have thriven for weeks had one 

 decided to rear it. Corresponding to the mass of yolk attached 

 to the embryos in Figs. 5 and 17, the older specimen had a 

 yolk sac of remarkably small size ; the sac was complete, however, 

 and its vitelline circulation resembled closely that of a shark. 

 Furthermore, it may be noted that a young CJdmcsra when 

 newly hatched has no trace of an external yolk-sac. I should 

 record, in this connection, that my friend, Dr. Wilbur, had already 

 (1898) observed on several occasions that the embryo with its 

 blastoderm appropriated only a portion of the entire egg mass, 

 and that the yolk-sac was but a miniature of a shark's. He 

 then believed, like myself, that such a remarkable condition 

 was abnormal. If a normal condition, however, the small yolk- 

 sac and the fragmental yolk bear evidently upon the question 

 of the total segmentation of this yolk-filled egg. And if the 

 evidence is conclusive which the present observations afford, 

 there is here given the first example of a large egg to undergo 

 holoblastic cleavage - - an interesting denial of the corollary 

 of ' Balfour's law,' that the quantity of yolk present in an 

 egg determines its holo- or meroblastic character. It may 

 be well, therefore, to review in ( this connection the evidence of 

 total cleavage in Chimcera. (i) The entire egg undergoes a 

 progressive fragmentation, in course of which fissures first pass 

 from the germinal region downward, and finally divide up the egg 

 into a series of yolk masses. (2) The foregoing process is a 

 normal one, having been observed in all specimens (later stages) 

 examined --a dozen or thereabouts. (3) On the evidence of 

 earlier stages distinct fissures (vacuolar) come to be formed in 

 the sub- and circum-germinal yolk, and these are shown to be 

 in many cases continuations of intercellular spaces of the germ 

 itself. (4) The yolk tracts separated by (vacuolar) fissures con- 

 tain nuclei ; this we may conclude from the conditions pointed 

 out in Figs. 10, loA, and from the presence of nuclei far out 

 beyond the germinal area (Fig. 10). In short, the egg actually 

 undergoes total division, following a nucleation and fissuring of 



