DEVELOPMENT OP PERIPATUS NOV^-ZEALANDI^. 247 



behind the anus in the middle line there is a mass of undif- 

 ferentiated cells (fig. 22, pr. st.), which is the primitive streak, 

 a groove running down its centre being the primitive groove 

 (fig. 22, pr. gr.). This is the earliest stage at which these 

 structures are present, and the cells do not appear to be in a 

 state of great activity. 



The Mesoderm. — The somites are present as a series of 

 paired hollow cavities on each side of the embryo. Their inner 

 or splanchnic walls are very thin (figs. 21 b and 21 c, Sp.), but 

 their outer are much thickened, and are composed of several 

 layers of nuclei (figs. 21 b and 21 c, So.). In the anterior somites 

 (fig. 21 b) the somatic wall has pushed out the thickened over- 

 lying ectoderm so as to form protuberances, which are the rudi- 

 ments of the legs. A slight ventral outgrowth of the somatic 

 wall towards the ectoderm is the rudiment of the duct of the 

 segmental organ (fig. 21 b, n. d.). The posterior pair of somites 

 communicate with one another across the middle line, and 

 their posterior walls are fused with the undifi'erentiated tissues 

 of the primitive streak. 



The endoderm is present in the form of a layer of fairly 

 regularly arranged nuclei lying in a protoplasmic reticulum 

 round the periphery of the yolk (figs. 21 a — c, End.). There 

 are a few nuclei in the central yolk, which latter is much less 

 voluminous than in previous stages. 



General Considerations. 



The Segmentation. — So far as my material has allowed 

 me to investigate the subject, the segmentation of Peripatus 

 novse-zealandise resembles closely that which has been 

 described in some other Arthropoda. Quite lately Henking,^ 

 among others, has described the segmentation in the eggs of 

 certain Phalangidse, and his account in many respects agrees 

 with that which I have given, noticeably in the formation of 

 the blastoderm and in the irregular arrangement in the young 

 stages of the nuclei composing it. The segmentation of the 



' Henking, H., " Untersucbungen viber die Entwicklung der Phalangiden," 

 Theil i, ' Zeit. fiir wissen. Zool.,' xlv, 1886, 



