RANA. 359 



hence the segmentation is total and unequal, the hypoblast half being 

 retarded. A modified blastula is, however, still produced. In the 

 skate the bulk of yolk is so enormous that only the epiblast cells and a 

 portion of the hypoblast can segment. Hence the segmentation is only 

 partial or meroblastic, producing, not a true blastula, but a cap or 

 blastoderm resting on a mass of unsegmented hypoblast and yolk. 



Gastrula. — The yolk in the frog is already sufficient to prevent the 

 normal archiblastic invagination, and as the hypoblast is too bulky to- 

 be tucked into the epiblast, the epiblast perforce extends round the 

 hypoblast, producing finally a small blastopore in the same position as 

 that oi Amphioxus, viz., at the postero-dorsal part of the embryo. In 

 the skate this is carried still further, and the rim of the epiblast has to 

 extend so far round and over the enormous mass of yolk that the 

 embryo differentiates during the process. The final result is the same 

 as before, the epiblast eventually meeting round a small blastopore at 

 the posterior end of the embryo. 



But in both the frog and the skate it is doubtful how far the archen- 

 teron is produced by true invagination. Probably in both cases it 

 arises mostly by a split in the hypoblast. In the frog the archenteron 

 is largely filled by a ventral mass of yolk-cells, but in the skate this is 

 so enormous that it protrudes as a large separate mass of the body. 



Chordula. — The frog embryo, a day or two before hatching, as has 

 been seen, can be directly compared with the chordula larva, but here 

 again there are modifications. The notochord is not folded off, but 

 arises as a solid mass, and the mesoderm no longer arises as a paired 

 series of pouches with cavities in continuity with that of the gut, but as 

 solid masses, with cavities produced later by splitting. 



In the skate much the same as in the frog occurs, if we consider the 

 embryonic portion only. We may note that both the frog and skate 

 appear to have at least the main part of the mesoblast formed from two 

 posterior sheets or plates comparable to the posterior sacs oi Amphioxus 

 and of Ascidia. Again, in Amphioxus the whole of these sacs divide 

 into mesoblastic somites, the ventral parts of which fuse later to form 

 the perivisceral coelom, the dorsal parts remaining segmented. On the 

 other hand, in the skate and frog the ventral part or lateral plate is never 

 segmented, but splits at once to form the perivisceral coelom, only the 

 dorsal part or vertebral plate being segmented, as in A7nphioxus. The 

 result in all three types is the same though brought about in a different 

 manner. 



Fcetal Membranes. — In the frog the yolk distends the abdomen, 

 but is not sufficient to cause the formation of a complete yolk-sac. In 

 the skate, however, the yolk is so abundant that the embryo cannot 

 possibly be built up to include the yolk, and the latter has to be held in 

 a special sac. The outer wall of this sac is the serous membrane, a 

 continuation of the body- wall, and the inner is the yolk-sac proper, 

 a continuation of the gut-wall. Hence we see that in the skate the 

 abundance of yolk (and lecithal nutrition) has caused the formation of 

 two extra-embryonic foetal membranes, the serous membrane and the 

 yolk-sac membrane. In all the Ainniota not only are these two present, 

 but two more, the amnion and the allantois, are superadded. 



