128 EARLY STAGES IN THE 



negative. It is quite easy to conceive the food material of the 

 Frog's egg completely vanishing, but although this would entail 

 simplifications of development and possibly even make seg- 

 mentation uniform, there would, as far as I can see, be no 

 cause why the essential features of difference between the 

 Frog's mode of development and that of Amphioxus should 

 change. The asymmetrical and slit-like form of involution on 

 the one side and the growth of the epiblast over the mesoblast 

 on the other side, both characteristics of the present Frog's egg, 

 would still be features in the development of the simplified egg. 



In the Mammal's egg we probably have an example of a 

 Reptile's egg simplified by the disappearance of the food ma- 

 terial ; and when we know more of Mammalian embryology it 

 will be very interesting to trace out the exact manner in which 

 this simplification has affected the development. It is also pro- 

 bable that the eggs of Osseous fish are fundamentally simplified 

 Selachian eggs ; in which case we already know that the dimi- 

 nution of food material has affected but very slightly the funda- 

 mental features of development. 



One common feature which appears prominently in reviewing 

 the embryology of vertebrates as a whole is the derivation of the 

 mesoblast from the hypoblast ; in other words, we find that it is 

 from the layer corresponding to that which becomes involuted 

 in Amphioxus so as to line the alimentary cavity that the meso- 

 blast is split off. 



That neither the hypoblast or mesoblast can in any sense be 

 said to be derived from the epiblast is perfectly clear. When 

 the egg of Amphioxus is in the blastosphere stage we cannot 

 speak of either an epiblast or hypoblast. It is not till the invo- 

 lution or what is equivalent has occurred, converting the single- 

 walled vesicle into a double-walled one, that we can speak of 

 these two layers. It might seem scarcely necessary to insist 

 upon this point, so clear is it without explanation, were it not 

 that certain embryologists have made a confusion about it. 



The derivation of the mesoblast from the hypoblast is the 

 more interesting, since it is not confined to the vertebrates, but 

 has a very wide extension amongst the invertebrates. In the 

 cases (whose importance has been recently insisted upon by 

 Professor Huxley), of the Asteroids, the Echinoids, Sagitta, and 



