GENERAL EMBRYOLOGY, 



157 



used ; these names are strictly applicable only to those eggs with discoidal 

 cleavage. In the bird's egg, for example, the two germ-layers form over 

 the unsegmented yolk, from which they become separated by the gas- 

 trular cavity ; thus, then, the external germ-layer actually lies above, the 

 internal below. Other terms for the two germ-layers are entoblast and 

 ectoblast. 



Delamination. In regard to the mode of development of the gastrula 

 many controversies have arisen which are not yet finally settled ; in addi- 

 tion to invagination there may exist a second, but very much less frequently 



J? ^ ****** C 



FK;. 106. Delamination of the egg of a Geryonid. (After Fol, from Korschelt- 

 Heider.) ft, cleavage cavity ; 0, jelly. 



occurring mode of development, delamination. In delamination the 

 blastula may become two-layered by tangential division of its cells (fig. 

 106) ; each single blastoderm-cell, or, at least, the majority of these cells, 

 by this division falls into a peripheral ectodermic and a central ento- 

 dermic cell. In case of delamination the cleavage cavity becomes directly 

 the cavity of the digestive track, a fact which renders it, difficult to 

 regard delamination and invagination as modifications of one and the 

 same process. 



Formation of the Mesoderm. The Mesenchyme. Many lower 

 animals, e.g. most coelenterates, 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 further differentiation 

 begins, there arises still a third germ-layer, which, owing to its 

 position between the first two, is called the mesoderm, mesoblast, 

 or middle germ-layer; this naturally can come only from the cell 

 material of the existing germ-layers, and indeed only the entoderm 

 seems to participate in it. Two methods can be distinguished in 

 its formation. In one the space between ectoderm and entoderm 

 becomes widened by the secretion of gelatinous substance, and 

 from the entoderm isolated cells push into this jelly; thus there 

 arises a middle layer, the mesenchyme (fig. 107), somewhat similar 



