DEVELOPMENT OF THE CAPE SPECIES OF PERIPATUS. 531 



corresponding with the nuclei in the lips of the blastopore* 

 intermediate in character as well as in position^ between the 

 ectodermal and endodermal nuclei. The multiplication of these 

 nuclei gives rise to a primitive streak, which, as in the Verte- 

 brata, is entirely posterior to the blastopore, and is marked by 

 a longitudinal groove — the primitive groove. 



This process resembles, in all essential points, the formation 

 of the greater part of the mesoderm in other Tracheata from 

 the walls of the germinal groove, diflFering only in this, that 

 whereas in the latter the germinal or primitive streak occupies 

 the greater part of the ventral surface, in Peri pat us it is 

 confined to the part of the ventral surface behind the anus. 



I have elsewhere (No. 32) stated my reasons for agreement 

 with Balfour^s view, viz. that such a method of mesoderm 

 formation is probably to be regarded as a modification of 

 archenteric diverticula, such as are found in Amphioxus, &c. 

 Whether the origin of mesoderm from the walls of archenteric 

 diverticula is a primitive process or not is open to grave 

 doubt. 



It seems to me there is a large body of embryological facts 

 which suggest, at any rate, the view that the mesoderm arose 

 as a result of the differentiation and rearrangement of certain 

 of the nuclei of the amoeboid central mass of the ancestral 

 parenchymella or gastrula; that is to say, the facts seem 

 to suggest the following as a possible general view of the origin 

 of the three layers of the Triploblastica. 



(«) Starting with a large multinucleated Protozoon, the 

 first advance consists in the differentiation of a cortical layer 

 of nuclei and of the protoplasm governed by them into a 

 peripheral layer or ectoderm. This layer was possibly of 

 a plastic nature, and allowed the protrusion of the central 

 mass at one or more points. The central mass would, in 

 consequence of its large size, probably be capable of arrang- 

 ing its vacuoles into a series of thoroughfares through itself 

 from one opening on the surface to another, so that the intro- 

 duction of nutritive matters to its deeper parts would be possible. 

 On the analogy of the Platyhelininth excretory system we may 



