THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 175 



2. The Development of the External Form of the Body. 



In spite of the variations found in the first ontogenetic stages of 

 the several species of Peripatus, the latter differ very little from one 

 another in the development of the external form of the body. In 

 the following descriptions we shall first deal chiefly with P. cajpensis, 

 which was very carefully examined first by Moseley (No. 6), then by 

 Balfour (No. 1), and later by Sedgwick (No. 10, Pt. i.). 



P. capensis. It has already been shown, in describing the forma- 

 tion of the germ-bands, that a thickening of the blastoderm arises 

 behind the lengthening blastopore by the proliferation of cells, this 

 spot being recognisable externally as an oval area (Fig. 83). We 

 saw that at this point the mesoderm originates, and extends forwards 

 in the form of two bands, to the right and left of the blastopore. 

 In each mesoderm-band segmentation takes place, a segmentation 

 which may in all respects be compared to that of the Annelida. 

 For instance, at the anterior ends of the two bands, cell-complexes 

 are cut off and commence, by the formation of cavities, to form the 

 primitive segments (Fig. 84 A and B), fresh rudiments being 

 continually added posteriorly. At the posterior end of the blasto- 

 pore the mesoderm-bands pass over into an undifferentiated cell-mass. 



During the differentiation of the mesoderm-bands another im- 

 portant change takes place in the embryo. The lips in the middle 

 region of the elongated blastopore approach one another and fuse, 

 so that the only remains of the blasto- 

 pore are an anterior and a posterior 

 aperture (Fig. 84 A and B). These 

 two apertures are henceforward retained 

 (C and D), giving origin (in connection 

 with ectodermal invaginations) to the 

 mouth and the anus. 



The next changes in the embryo 

 take place as follows : as the differ- 

 entiation of the primitive segments ^--~- -- 



continues, the first of these shift Fig. 83.— Embryo of P. capensis (after 

 further forward, and, in addition to Balf0 ™)- * blastopore ; w, zone 



' of growth. 



this internal segmentation of the em- 

 bryo, an outer segmentation now appears (Fig. 84). At the anterior 

 end the cephalic lobes begin to appear, and it is to be specially noted 

 that, as rudiments, they show great resemblance to the body-segments. 

 The posterior end of the hitherto straight embryo curves round 



