256 SOME RECENT 



formation of the inner germinal layer, or hypoblast, 

 presents most perplexing modifications : it may 

 arise as a true gastrula invagination ; as cells 

 budded off from one pole of the blastula into its 

 cavity ; as cells budded off from various parts of 

 the wall of the blastula ; by delamination, or actual 

 division of each cell of the blastula wall into outer 

 or epiblastic, and inner or hypoblastic elements ; 

 or it may be present from the first as a solid mass 

 of cells enclosed by the epiblast cells. Another 

 good illustration is afforded by the extraordinary 

 modifications in the position, and in every detail of 

 formation of the middle germinal layer, or mesoblast, 

 in different and often in closely allied forms of the 

 higher Metazoa : differences which have given rise 

 to ardent discussion, and have led to the proposal 

 of theory after theory, each rejected in its turn as 

 affording only a partial explanation, and finally 

 culminating in Kleinenberg's protest against the 

 use of the term mesoblast at all, at any rate in a 

 sense implying any possibility of comparison with 

 the primary cellular layers, epiblast and hypoblast, 

 of Coelenterata. Amongst Vertebrates the frog and 

 the newt differ greatly in important developmental 

 points. The stages immediately following segmen- 

 tation of the egg are very unlike in the two cases. 

 The epiblast is but one cell thick in the newt, while 

 in the frog it consists of two distinct layers from 

 the first ; the ear of the newt develops as a pit- 

 like depression of the skin, while in the frog it is 

 from the first a closed sac ; and many other differ- 

 ences will readily occur to embryologists. In the 



