THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 29 



Peripatus and Myriapods, from the absence of wings and other primitive char- 

 acters, may fairly be considered the most primitive tracheates. 



The position of Peripatus is uncertain ; but some Myriapods show indications 

 of a hexapod stage in their development. They may therefore be related to the wing- 

 less Hexapods. The large number of body segments and appendages in Myriapods, 

 and perhaps in Peripatus as well, is probably only the vegetative reproduction of 

 homologous parts. 



In Myriapods and in Peripatus as shown by the studies of Kennel" 'and 

 Sedgwick ( ' 2> on the latter, and by observations of other observers on the former, the 

 segmentation is total. From the accounts given by Sedgwick and Kennel it appears 

 that the gastrulatiou differs in different species of Peripatus. 



The mode of origin of the endoderm is not, however, very important for classi- 

 ficatory purposes, inasmuch as it is very likely to be modified by the presence or 

 absence of food yolk. 



The mesoderm, in the development of Peripatus, grows forwards from an 

 undifferentiated cell mass at the posterior end of the embryo. The mesoderm 

 arising from the primitive cumulus of spiders also grows forwards from an undiffer- 

 entiated cell mass at the posterior end of the embryo. 



The endoderm of Peripatus, Myriapods and Spiders is derived from the inner 

 layer of the gastrula. How the inner layer of the gastrula arises is unimportant. 

 Consequently Peripatus and spiders are quite alike in the formation of germinal 

 layers. 



This alone, however, does not indicate any close relationship, for in Crustacea 

 the endoderm arises from the inner layer of the gastrula while the mesoderm grows 

 forward from the posterior end of the embryo as in Peripatus and spiders. 



In the higher insects the yolk cells, from their mode of origin, probably repre- 

 sent the inner layer of the gastrula and are consequently equivalent to the endoderm 

 of lower forms. The true endoderm is functional only during embryonic life in 

 absorbing the yolk. It takes little or no part in the formation of the digestive 

 tract. In these higher insects, as already shown, the inner layer, which from its 



(1) Kennel. Entwicklungsgeschichte der Peripatus Edwardsii. Sempers' Arbeiten 1884. 



(2) Sedgwick, Development of Peripatus Capensis. Quarterly Journal, 1885. 



