ORIGIN OF THE GERM IX AL LAYERS. 275 



who would venture 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 investi- 

 gated, 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 Reraak (No. 287), who succesc- 

 fully worked out the problem of vertebx'ate 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 com- 

 pleted till after its termination. Not only do the 

 cells of the blastoderm become differentiated into « 



two layers, but these two layers, in the case of 

 a very large number of ova with but little food- 

 yolk, constitute a double- walled sack — the gastrula 

 (fig. 198) — the characters of which are too well 

 known to require further description. Following 

 the lines of phylogenetic speculation above in- 

 dicated, it maybe concluded that the two-layered 

 condition of the organism represents in a general 

 way the passage from the protozoon to the me- 

 tazoon condition. It is probable that we may 



safely go further, and assert that the gastrula rt;- Fi»- 19^- I>iagram 

 ^ ] -.1 1 c 1 Ti J- • OF A (tastkula. (From 



produces, with more or less fidelity, a stage in GeKenbaur.) 



the evolution of the Metazoa, permanent in the „ moiith- h. ar- 



simpler Hydrozoa, during which the organism was chenteron ; c. hypo- 

 provided with (1) a fully developed digestive cavity Wast; d. epiblast. 

 (fig. 198 b) lined by the hypoblast with digestive 

 and assimilative functions, (2) an oral opening {a), and (8) a supor- 



18—2 



