20 



PERIPATUS 



uiit in one species, and to that species P. capensis the follow- 

 ing description refers. The ova are apparently fertilised in the 

 ovary, and they pass into the oviducts in April and May. In 

 May the brood of the preceding year are born, and the new ova, 

 which have meanwhile undergone cleavage, pass into the uterus. 

 There are ten to twenty ova in each uterus. The segmentation 

 is peculiar, and leads to the formation of a solid gastrula, consisting 



of 



mass, which consists of a much -vacuolated 



a cortex of ectoderm nuclei surrounding a central 



endodermal 

 tissue with some^ 



Fig. 13. A series of embryos of P. capensis. The hind end of embryos B, C, D is 

 uppermost in the figures, the primitive streak is the white patch behind the blasto- 

 pore. (After Sedgwick.) A, Gastrula stage, ventral view, showing blastopore 

 B, Older gastrula stage, ventral view, showing elongated blastopore and primitive- 

 streak. C, Ventral view of embryo with three pairs of mesoblastic somites, dumb- 

 bell-shaped blastopore and primitive streak. D, Ventral view of embryo, in which 

 the blastopore has completely closed in its middle portion, and given rise to two 

 openings, the embryonic mouth and anus. The anterior pair of somites have- 

 moved to the front end of the body, and the primitive groove has appeared on the 

 primitive streak. E, Side view of embryo, in which the hind end of the body has- 

 begun to elongate in a spiral manner, and in which the appendages have begun. 

 At, antenna ; d, dorsal projection ; jj.s, jjreoral somite. F, Ventral view of head of 

 embryo intermediate between E and G. The cerebral grooves are wide and shallow.. 

 The lips have appeared, and have extended behind the openings of the salivary 

 glands, but have not yet joined in the middle line. At, antennae; eg, cerebral 

 groove ; j, jaws ; j.s, swelling at base of jaws ; L, lips ; 3f, mouth ; o?\^j, oral papillae ; 

 o.s, opening of salivary gland. G, Side view of older embryo with the full number 

 of appendages, to show the position in which the embryos lie in the uterus. 



irregularly-shaped nuclei. The endoderm mass is exposed at one^ 

 point the blastopore (gastrula mouth). The central vacuoles 

 of the endoderm now unite and form the enteron of the embryo, 

 and at the same time the embryo elongates into a markedly oval 

 form, and an opacity the primitive streak appears at the hind 

 end of the blastopore (Fig.' 13, B). This elongation of the embryo^ 

 is accompanied by an elongation of the blastopore, which soon 

 becomes dumb-bell shaped (Fig. 13, C). At the same time the 

 mesoblastic somites (embryonic segments of mesoderm) have made 



