GENERAL EMBRYOLOGY. 



189 



not yet finally settled ; in addition to invagination there 

 may exist still a second, but at all events, a very much less 



Fiu. 102. Delamination of the egg of a Geryonid. (After Fol ; from Korschelt-Heider.) /t, 



cleavage cavity ; , jelly. 



frequently occurring mode of development, delamination. 

 In delamination the blastula may become two-layered by 

 tangential division of its cell's (Fig. 102); each single blas- 

 toderm-cell, or, at least, the majority of these cells, by this 

 division falls into a peripheral ectoblastic and a central en- 

 toblastic cell. In case of delamination, the cleavage cavity 

 becomes directly the cavity of the digestive tract, a fact 

 which renders it difficult to regard delamination and in- 

 vagination as modifications of one and the same process. 



Formation of the Mesoblast. The Mesenchyme. 

 Many lower animals, e.g. most cvlcntcratcs, have in general 

 only two germ-layers. When these are laid down there 

 begins immediately the differentiation of muscle and nerve 

 fibres and the other processes of histological changes 

 of the cells, as well as a series of changes of form, by 

 which the gastrula becomes the adult animal. In higher 

 organisms, on the other hand, before organological and 

 histological differentiation begins, there arises still a third 

 germ-layer, which, owing to its position between the first 

 two, is called the uicsoblast or middle gcrui-laycr ; this 

 naturally can come only from the cell-material of the ex- 

 isting germ-layers, and indeed only the entoblast seems to 

 participate in it. Two methods can be distinguished in 

 the formation of the middle germ-layer. In one case, the 

 space between ectoblast and entoblast becomes widened by 

 the secretion of jelly, and from the entoblast isolated cells 



