CLEAVAGE AND FORMATION OF GERM-LAYERS. 



41 



There is little agreement among authors concerning the onto- 

 genetic processes which follow the formation of the blastoderm, 

 some ascribing great significance to the prominence, called by 

 Claparede the primitive cumulus, which appears in the blastoderm 

 by the thickening of the cell-layer * (Figs. 22 B and 23 A and B), 

 others denying its importance. According to Morin, a thickening 

 of the blastoderm arises in the region which corresponds to the later 

 ventral surface, i.e., to the rudiment of the germ-band (Fig. 21 F) ; 

 not only do the cells here increase in size, but some of them 

 separate from the blastoderm, and form definite layers ; the blasto- 

 derm thus becomes multilaminar. At the same time a few cells 



a. 



&.■ 



Fig. 22.— Sections through the egg of Pholeus plialangioides during the formation of the germ- 

 layers (after Morin). c.p. primitive cumulus ; d, yolk ; dz, yolk-cells ; e, point of ingrowth. 



in this region become entirely disconnected from the rest, and 

 migrate into the yolk (Fig. 21 F, dz). The three germ-layers may 

 be now recognised. An outer layer, which constitutes the greater 

 part of the blastoderm, is the ectoderm. Below this, at one pole 

 of the egg, is the mesoderm, while the cells which have migrated 

 into the yolk represent the entoderm. In the Araneids observed 

 by Morin, the primitive cumulus arose only after the germ-layers 

 had formed, if indeed it arose at all. It was wanting in Theridium, 

 the form in which the origin of the germ-layers has been just 



* [Unfortunately there is a good deal of confusion surrounding the term 

 "primitive cumulus." As stated above, there are two thickened white areas 

 in the blastoderm ; and according to Kishinotjye, Claparede overlooked the 

 first of these, and applied the term " primitive cumulus " to the second. The 

 former author terms them the primary and secondary thickenings ; but 

 Kingslet, while agreeing that Kishixouye may be right, nevertheless retains 

 Clai'Arede's term " primitive cumulus" for the first thickening. — Ed.] 



