226 



INVEETEBEATA 



CHAP. 



appears considerably later, just in front of the tail-lobe, as a similar 

 invagination. 



When we compare the development so far recorded with that of 

 Peripatus we must be struck with the fundamental resemblance. 

 If, when the yolky endoderm cells of Peripatus have become invested 

 by the ectoderm, the gut-cavity— which, it must be remembered, is 

 formed by a process of secondary absorption— did not appear, then 



we should have a sohd yolky 

 endoderm, such as, to judge 

 from the few observations that 

 have been made on the subject, 

 in point of fact appears to exist 

 in Peripatus novae - zelandiae. 

 (Sheldon, 1888, 1889.) 



The origin of the mesoderm 

 as a pair of bands which become 

 divided into somites, each con- 

 taining a coelomic cavity, and 

 the origin of the whole central 

 nervous system as two ridges 

 united in front of the mouth in 

 the region of brain, and behind 

 the position where the anus 

 appears as an anal ganglion, is 

 essentially the same both in the 

 Spider and Peripatus. In the 

 cephalic lobe of the Spider on 

 each side there is a faint spht in 

 the mesoderm, which seems to be 

 the rudiment of a coelomic cavity. 

 Whilst all this development 

 has been going on, the ventral 

 plate has grown in length till it 

 almost encircles' the ovum, the 

 part of the circumference which 

 represents the dorsal surface 

 being of very small extent, and 

 the caudal and cephalic lobes 

 almost touching one another. Underneath this small dorsal surface 

 there appear at the same time a number of cells gorged with yolk 

 (" fat " cells). Previous observers had interpreted these to be wander- 

 ing mesoderm cells and the forerunners of blood corpuscles, but 

 Kishinouye interprets them as endoderm cells now for the first time 

 reaching the dorsal surface of the yolk, though he admits that they 

 develop into the first blood cells. Kautsch points out that at no 

 time "is it possible to discriminate sharply between mesoderm cells 

 and yolk cells, since all intermediate stages between these two types 

 occur. 



Fig. 173. — Two sagittal sections through em- 

 bryos of Agelena of different ages, but 

 previous to reversion. (After Kishinouye. ) 



A, younger stage. B, older stage. Letters as 

 in previous figure, ccoe, cephalic coelom. In A 

 no coelomic sacs have appeared in the cephalic lobe 

 or in the cheliceral segment. In B the vestige of 

 an appendage on the sixth abdominal segment 

 present in A has disappeared. 



