GROWTH OF THE MAMMALIAN GASTRULA. 287 



In Mammals the corresponding germinal processes are 

 very complex and peculiar. Till quite recently they were 

 entirely wrongly explained ; the recently published researches 

 of Eduard van Beneden 69 which placed them, for the first 

 time, in a true light, enabled us to bring them into agree- 

 ment with the principles of the Gastrsea Theory, and to trace 

 their relation to the germination of the lower Vertebrates. 

 Although there is no independent nutritive yelk, distinct 

 from the formative yelk, in the mammalian egg, and 

 although the cleavage is therefore total, a large yelk-sac 

 arises from the embryo which is produced by this cleavage, 

 and the true germ spreads itself in a layer-like form on the 

 surface of this yelk-sac, as in the case of Reptiles and Birds, 

 the eggs of which have a large nutritive yelk and undergo 

 partial cleavage. As in the latter, the flat, leaf-shaped 

 germ-disc of Mammals detaches itself from the yelk-sac, 

 its walls incline towards each other and coalesce into tubes. 



This striking contradiction can only be explained as a 

 consequence of very peculiar, strange, kenogenetic modi- 

 fications of the germ, the causes of which are not yet fully 

 explained. They are evidently connected with the fact 

 that the ancestors of the viviparous Mammals were Amnion- 

 animals, which laid eggs, and which only gradually became 

 viviparous. When the Hood-gastrula (Amphigastrula) of 

 the Mammal is complete (Fig. 71), it changes into a large 

 globular vesicle, filled with fluid. According to Van 

 Beneden, this happens in the following way : The Gastrula- 

 mouth disappears in consequence of the entoderm-cell (0), 

 which formed the yelk-plug, disappearing into the interior, 

 to the other cells of the intestinal layer (d). The mam- 

 malian germ now forms a solid ball, consisting of a quantity 



