212 WALTBE HE APE. 



one larger and hyaline, the other smaller and containing a 

 more dense vitelline material. The hyaline segment he calls 

 the epiblastiCj the more opaque segment the hypoblastic 

 sphere. He then describes the order of the subsequent seg- 

 mentation phenomena, and declares that the segments derived 

 from the primary hyaline epiblastic sphere gradually grow 

 round those formed from the primary hypoblastic sphere, and 

 there results a structure precisely similar to that described 

 above (p. 210), which he calls the " metagastrula" stage. This 

 metagastrula Beneden compares with the gastrula of lower 

 types, and he derives the epiblast of the blastodermic vesicle 

 and of the embryo from the outer "epiblastic" spheres, and 

 the hypoblast and a portion of the mesoblast from the inner 

 " hypoblastic spheres." 



There can be little doubt, however, that Beneden's account 

 of the derivation of the layers is incorrect, and that the greater 

 portion of the inner segments, as well as the whole of the outer 

 segments, give rise to epiblast. When this is considered, and 

 when the probable homologies of the primitive streak are recol- 

 lected, any comparison of the so-called "metagastrula" of the 

 Mammalian ovum with the gastrula of lower types is found to 

 be impossible, and the significance of whatever differences may 

 exist in the two primary segments is rendered unimportant. 



In the absence of any figures in Beneden's paper I have been 

 unable to compare the appearance of the segments he describes 

 in the Rabbit's ovum with those I have examined in the Mole, 

 but I have myself examined segmenting ova of the Rabbit, and 

 have isolated the segments the one from the other, in order the 

 more clearly to compare them, and in no case have I been able 

 to distinguish the slightest difference in the density or con- 

 stitution of these segments. 



If my observations are correct, then, the differentiation of 

 the segmentation spheres into two layers in the fully segmented 

 ovum is not a primary differentiation such as Beneden discerns, 

 but a secondary differentiation due to the peculiar circum- 

 stances of nutrition and development attending the formation 

 of the Mammalian embryo. 



