THE DEVELOPMENT OF VERTEBRATA. 263 



4. Explanation of the Morula. Now the explanation 

 which naturally suggests itself is, that we have here a 

 gastrula, and that invagination has been hurried forward, 

 as it were, so that the blastosphere stage has been skipped. 

 On this view (adopted by Professor Yan Beneden, who first 

 described the segmentation of the rabbit's ovum) the outer 

 layer is epiblast, the inner primitive hypoblast, and the 

 point where the latter is exposed is the blastopore. Un- 

 fortunately for this view, later development absolutely 

 contradicts it. The outer layer takes no part in the 

 formation of the embryo, but is purely extra-embryonic. 

 The whole embryo (epi-, meso-, and hypoblast) is formed 

 from the inner cells ; and Van Beneden's blastopore, as it 

 is still called, in no way corresponds with the blastopore of 

 Amphioxus or frog. 



Thus direct comparison with Amphioxus leads us astray ; 

 but comparison with the fowl is more hopeful. Imagine a 

 series of eggs intermediate in character between that of 

 the bird and that of the mammal. As the ova got 

 successively smaller, we should find the blastoderm en- 

 circling the yolk more and more quickly; while as yolk 

 was withdrawn from the centre, the central part of the 

 blastoderm would sink into the hollow thus produced, By 

 the time we reached an ovum with practically no yolk, 

 we should find the area pellucida sunk into the centre of the 

 ovum, and the area opaca closely surrounding it, just as 

 the outer layer of cells does actually surround the inner 

 layer in the rabbit's segmented ovum. 



Whether this is precisely the true explanation or not is 

 uncertain, because the difficulties of investigation have so 

 far prevented the tracing of the fate of the outer layer 

 fully enough. Possibly it may be an entirely new structure, 

 not represented in the chick. 



5. The Blastodermic Vesicle. The next stage (fig. 

 139 [2]) shows the appearance of a space between the two 

 layers, away from Van Beneden's blastopore. This space 

 becomes filled with secreted fluid, and grows larger and 

 larger, as the outer layer becomes thinner and more 

 extensive, until at last we find the inner layer cells forming 



