62 



K. KISHINOUTE. 



the existence of a blastodermic thickening giving rise to germinal 

 layers, but denied the identity of it with the primitive cumulus. He 

 says that the primitive cuniolus is formed after the formation of the 

 germinal layers and is composed of mesoderm cells. My observations 

 on Lycosa show that the secondary thickening, or the primitive 

 cumulus of Claparède, is formed after the formation of the primary 

 thickening and that both are formed before the distinction of ger- 

 minal layers is possible. lîoth are accumulations of indifferent cells, 

 not yet referable to any germinal layer (PI. XII, figs. 11-14). I can 

 not tell whether the position of the secondary thickening corresponds 

 to the anterior or to the posterior (.)f the future ventral plate. This 

 much is certain, that it entirely disappears at the time when the 

 germinal layers are established. 



These two thickenings, the primary and secondary, are of a 

 great significance, as the germinal layers are established from them, 

 the primary thickening contributing the largest part in their forma- 

 tion. In a longitudinal section, these two thickenings are as in figs. 

 12 and 14, while in a cross section they appear as in fig. 11. From 

 these figures it is evident that they together form along the median 

 ventral line of the future embryo a ridge-like thickening which sticks 

 out into the cavity of the yolk. Cells from the top of this ridge 

 (the lowest part of the ridge in the figures) proliferate into th*e yolk 

 and become scattered without any definite arrangement through the 

 entire yolk. These are the endoderm cells. They become large 

 by takiijg nourishment from the yolk as they pass through it. The 

 cell-layer of the ridge nearest the external face of the egg becomes 

 established as the definite ectoderm. The cells of the ridge 

 which are left close under the ectoderm form the mesoderm 

 (PL XIII, fig. 17). They soon spread horizontally below the ecto- 

 derm. The mesoderm is at first in a single median mass on the 



