DEYi:i.oi MI;M Of Tin: nfO l'i;iM\i;\ OBBM i.AM'.US. 01 



is sul>st (jut-iit ly tin- hind end (//)<>' t he emhryo, at a n Lri<>" i' tin- 

 /one nt' irarMtion >r inai-iriii of the g'rm-disr, in which the 

 snjn rficial cvlU have assumed tin- cylindrical I'm in. ami aiv <!> ly 

 joint (I io-vthtr (fig. 49). The mar-in of tin- di>c is fold d in 

 (ti_r. f)0) toward the cleavair -ca\ ity (l>). M> bhl .i!l cn-len- 



teron (>/), shown in tin- accompanying section, and a li- 

 like hla>t>pore are distinctly ivo:iii>all'. The m-Ldil.oiing yolk 

 al-o participates in tin- in\ airina t ion, >ince in the territory of 

 the /one of transition tin- yolk nnclci ('/), envelop* d in proiopl.-.sm. 

 U foine dt tachcd from tlie yolk, irroxv into the cleavage -cavity along 

 with the inxairinated ci 11s, and contribute to the formation of the 

 inner trerm- layer in a similar manner to that in whirh, in the ca 

 the Amphihia, the vegetative cells at the lower lip of the blastopore 

 are carried in with the imagination into the cleava: '-cavity. The 

 clea\ai:e-cavity (/>) is being continually encroached upon by the in- 

 growth of the cells originally in its roof, which form a continuous 

 layer projecting from behind forward. Consequently in the Sela- 

 chians also the germ-disc becomes two-layered as the result of the 

 in valuation. It lies so close upon the yolk, that the crelenteron 

 appears at most as a fissure. Moreover, the invagination in the 

 Selachians does not remain limited to one region of the original 

 margin of the germ-disc, but soon stretches itself out over its whole 

 posterior perimeter. The blastopore then appears as a large semi- 

 circular or horseshoe-shaped fissure at the future posterior end of the 

 -inhryonic fundament. 



The enormous volume of the yolk causes an important differ* 

 in t \\oen the gastrulation of the Selachii and that of the Amphibia. 

 In the case of the latter the mass of the yolk-cells was quite rapidly 

 carried in with the invagination, and employed in the formation of 

 the ventral wall of the co?lenteron. In the Selachians the taking 

 up of the yolk into the interior of the body ensues only at a slow 

 rate (in a manner to be more accurately explaimd later), so that for 

 a long time only the dorsal side of the gastrula consists of two cell- 

 layers, whereas the ventral wall is fornn d by the yolk-n 



The eggs of Teleosts are very in ni ly n lat. d to those of Selachians 

 in their whole method of development. The same cannot be- 

 to be true to the same extent for the eggs of Reptiles and 

 J'.irds. The latter, indeed, also belong to the meroblastic type, 

 since they have developed a large amount of yolk, and in c.ons (jiience 

 undergo partial segmentation; but in the formation of the germ- 

 lay, is, they exhibit many peculiarity s, so that they require a separate 



