ee 



K. KISHTNOUYE. 



know these invaginations have been till now entirely overlooked. 

 Til fig. 19, PI. XXI, of Balfoar's work, I find one of these in- 

 vaginati(jns represented ; but he gives no informati(3n about it. It 

 is globular in form ; henceforward I shall call it the lateral vesicle. 

 The lateral vesicles, which are also gradually constricted off frc^m the 

 ectoderm, go to form a part of the brain (PL XV, figs. 44-46). 



The stomodœum is formed as an ectodermic invnirination at the 

 anterior margin of the cephalic region (PI. XIII, fig. 20, PI. XIV, 

 figs. 24, 25). At this stage it is easy to see that all the appendages 

 are postoral in origin. 



Late in this stage a number of large cells appear at the dorsal 

 part of the embryo. They are never found in the ventral plate. 

 They are very easily recognised by their large size and the peri- 

 pherally situated nuclei, their central porti<3n being filled with fat 

 (PI. XV, figs. 40, 41,/. c). Undoubtedly they are nourishing cells, 

 wandering everywhere, and some of them are changed into blood 

 corpuscles. They were called by Balfour the secondary mesoderm, 

 l)y Schimke witch the secondary endoderm, and by Locy the endo- 

 derm cells. These three authors ascribed the oriu'in of tliese fit cells 

 to the cells in the yolk, whereas according to Morin they are formed 

 in Pholcus from dispersed mesoderm cells originally composing the 

 so-called primitive cumulus,* and in Theridion which wants the 

 cumulus probably from cells of the mesodermal somites. Schimke- 

 witch, Locy, and Morin observed that these cells become blood 

 corpuscles. For my own part, I am inclined to agree with Balfour, 

 Schimkewitch, and Locy and to derive them from the endoderm. 

 For in the first place, they are found immediately above the yolk, 

 arid in some cases between yolk granules presenting the appearance as 



* Morin states, what I have before referred to, that the primitive cumulus is formed after 

 tlio foruiation of germinal layers, and consists of mesoderm cells. 



