224 INVERTEBRATA CHAP. 



is the primitive cumulus, and that it appears at the hinder end of 

 the primitive streak. Amongst the cells budded off' from the primitive 

 cumulus are probably the mother cells of the genital organs. It 

 should be noted, however that Kautsch regards the so-called primitive 

 cumulus as a thickening of no special significance, and expressly 

 denies that it has anything to do with the genital organs he re- 

 gards it as separated from the front end of the primitive streak. 

 The difficulty is that it has ceased to be recognizable before the first 

 traces of segments have appeared in the embryo, before it has become 

 possible to determine the long axis of the embryo with certainty. 



After the secondary thickening has disappeared as a distinct mark, 

 the primary thickening extends so as to form, when seen from the 

 surface, an oval plate, which may be termed the ventral plate, on 

 which the first rudiments of the future organs make their appear- 

 ance. Transverse sections show that the proliferations of cells from 

 the primary thickening form a longitudinal keel-like ridge projecting 



into the yolk. This 

 keel -like ridge re- 



, : . 4%B . ""**\%5ft -, presents a solid in- 



'~^~&jr*&&2&&\ vaginatiori of endo- 



derrn. From the 

 edge of the keel, cells 

 are given oft 1 which 

 wander through the 



FIG. 171. Section through the primitive streak of Agelena to *. 



illustrate the formation of the germ layers. (After Kishinouye.) Slze as tney QO SO, 



and they eventually 



end, endodenn ; mes, mesoderm. , i T . , i 



establish an epithe- 

 lium on the outer surface of the yolk in the manner described below. 

 These cells constitute the permanent endoderm. The rest of the ridge 

 flattens out laterally and becomes divided into two parallel bands lying 

 beneath the ectoderm ; they constitute the mesoderm (Fig. 170). 



Soon, as seen from the surface, the ventral plate becomes crossed 

 by transverse grooves, which are the first indication of a division of 

 the animal into metamerically arranged segments. In front there is 

 an undivided cephalic lobe ; behind, a caudal lobe ; and the inter- 

 vening plate is divided, in the earliest stage observed by Kishinouye, 

 into five segments. Of these, the first corresponds to the segment 

 from which the second pair of appendages, the pedi palpi, develop; 

 the others correspond to the segments which bear the walking legs. 

 The segment which gives rise to the first appendages or chelicerae 

 develops later, being cut out from the cephalic lobe. 



As development proceeds, further segments are cut out from the 

 caudal lobe, till a total number of ten abdominal segments has 

 appeared. The mesodermic bands have meanwhile undergone a 

 corresponding segmentation into squarish blocks of cells, the somites, 

 in each of which a coelomic cavity occurs. The tenth segment is not 

 always distinct, nor does its somite always develop a cavity. Soon the 



