YOLK-SAC OF EARLY EMBRYO. lOI 



only, representing about one-tenth the bulk of the unsegmented egg, is reserved 

 for the yolk-sac of the young fish. 



In the stage last described (/. c\, of plate vn, fig. 44), in spite of the advanced 

 characters of the embryo, the blastoderm has not increased vastly in size beyond 

 that shown in plate v, fig. 38. It has, however, as we see in plate vin, fig. 47, 

 constricted marginally, becoming cup-shaped, as it continues to envelop the small 

 yolk mass. How far it has succeeded in inclosing the yolk is perhaps better seen 

 in the details of the last figure, shown in figs. 47'' and 47*". 



The relation of yolk and blastoderm is pictured in detail in fig. 77, a section 

 passing through the blastoderm parallel to the long axis of the embryo. At the 

 points m/> and w//, the rim of the blastoderm comes in contact with the yolk ; 

 above jud the blastoderm is thickened and 

 spongy; for, as a sign that the body of the 

 embryo lay adjacent, this region is richly 

 vascular. Noteworthy here is a deep sub- 

 marginal sinus (ms) whose posterior wall (/) i' 

 is cellular. We have in this condition a J 

 physiological parallel with the submarginal r P '- ,i 

 space in ganoids, and more directly even with -'^ ms 

 Kupffer's vesicle in teleosts. On the ventral '"^ i%^ ( ' i^l-'^ 

 side of the blastodermic cap (on the left in „/,'y'- ;.)~ml> 



the figure) the vascular sponginess is largely L, ' y 



lost; and the blastoderm is thin, save only at 



• , ' / t f\ All • 1 r J Fie. 77.— Section of extra-embryonic region ana of upper 



Its run imb ). And here m i-lace of a deep ^^^^ ^^ ^^,^ ^^^ ^, ^,^^^J^^^,^ viii. fig. 47. 



submarginal sinus, a number of distinct blood- ,.. OUaUrarea: mO, <»//. margin o( blastoderm: ms, marginal 



d- 1 / ' \ 4.4- „ J sinus ; /.y, fluid yolk ; I', vacuoles, 



ucmg vesicles appear {ms ) scattered 



distally in a narrow zone of finely divided yolk ( /r). From another standpoint, 



finally, the present section is noteworthy. For it shows that the entire yolk-sac is 



divided into masses which are largely separated from one another by a system 



of fissure-like vacuoles. Closer inspection shows nuclei scattered irregularly 



through these masses of yolk, and, everything considered, I think we can therefore 



justly conclude that the yolk-sac at this stage, in spite of its relatively large size, is a 



totally segmented structure comparable with the yolk-sac of Amia or Ichthyophis. 



In the present case, it is true, the yolk masses (blastomeres) show a condition of 



greater or less attachment to their neighbors, and each mass will usually contain 



more than a single nucleus. But even in this event, the comparison will, I believe, 



hold. In some cases the shape of the yolk masses is distinctly blastomere-like, 



as between the vacuoles (v) in the present section. Viewed from this standpoint, 



accordingly, Chimsera has retained a primitive embryological character, holoblastic 



cleavao-e- but we can hardlv fail to observe that this character has lost much of its 



primitiveness inasmuch as the blastomeres are polynuclear and the intercellular 



spaces obviously adapted as reservoirs of nutriment. 



