332 INVAGINATION. 



would be given to the questions which must necessarily be 

 raised in the present chapter ; but the results of the extended 

 investigations made during the last few years have shewn that 

 these expectations were premature, and in spite of the numerous 

 recent valuable contributions to this branch of Embryology, 

 amongst which special attention may be called to those of 

 Kowalevsky (No. 277), Lankester (Nos. 278 and 279), and 

 Haeckel (No. 266), there are few embryologists who would ven- 

 ture to assert that any answers which can be given are more 

 than tentative gropings towards the truth. 



In the following pages I aim more at summarising the 

 facts, and critically examining the different theories which can 

 be held, than at dogmatically supporting any definite views of 

 my own. 



In all the Metazoa, the development of which has been in- 

 vestigated, the first process of differentiation, which follows 

 upon the segmentation, consists in the cells of the organism 

 becoming divided into two groups or layers, known respectively 

 as epiblast and hypoblast. 



These two layers were first discovered in the young embryos of verte- 

 brated animals by Pander and Von Baer, and have been since known as 

 the germinal layers, though their cellular nature was not at first recog- 

 nised. They were shewn, together with a third layer, or mesoblast, which 

 subsequently appears between them, to bear throughout the Vertebrata 

 constant relations to the organs which became developed from them. A 

 very great step was subsequently made by Remak (No. 287), who success- 

 fully worked out the problem of vertebrate embryology on the cellular 

 theory. 



Rathke in his memoir on the development of Astacus (No. 286) at- 

 tempted at a very early period to extend the doctrine of the derivation of 

 the organs from the germinal layers to the Invertebrata. In 1859 Huxley 

 made an important step towards the explanation of the nature of these 

 layers by comparing them with the ectoderm and endoderm of the Hydro- 

 zoa ; while the brilliant researches of Kowalevsky on the development of 

 a great variety of invertebrate forms formed the starting point of the current 

 views on this subject. 



The differentiation of the epiblast and hypoblast may 

 commence during the later phases of the segmentation, but 

 is generally not completed till after its termination. Not 

 only do the cells of the blastoderm become differentiated 



