THE DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



177 



the amis (? = blastopore), of this undifferentiated cell-mass (primitive streak). 

 In the two first species this zone of growth is undoubtedly situated behind the 

 anus (i.e., behind the blastopore = anus in P. capensis, Fig. 84 A, iv, and in 

 the region of the blastopore and behind the anus in P. Edwards! i, Fig. 89 A), 

 whereas in P. novae-zcalaadiae this zone of growth appears, from a superficial 

 examination, to be situated in front of the anus (Fig. 86 A), at least the two 

 halves of the germ-band bend forward and unite in front of that aperture. 

 According to L. Sheldon this is onl}'' an apparent difference, for in sections of 

 P. novae-zealandiac through this region the zone of growth of the mesoderm 

 (primitive streak) is found to be situated, as in P. capensis and P. Edwardsii, 

 behind, not in front of, the anus. The possibility of the zone of growth being 

 situated in front of the anus is of interest 'when we make a comparison between 

 the embryos of Peripatus at this stage and those of the Annelida. Such a 

 comparison made between the embryo of P. novae-zealandiac (Fig. 86 A) and 

 that of Clepsine among the Hirudinea (Vol. i., Fig. 152, p. 322) reveals a 

 striking similarity between the two, especially in the configuration of the 

 mesoderm-bands which, in the Hirudinea, however, unite in front of the anus, 

 a condition which, it is true, is suggested from a superficial examination of the 

 embryo of P. novae-zealandiac, but which is not substantiated by the investiga- 

 tion of sections, and one which would, moreover, seem improbable from a 

 comparison with the two other species of Peripatus mentioned above. Further, 

 in the Hirudinea, there appears to be no connection between the blastopore and 

 the anus, which makes a comparison with Peripatus more difficult than would 

 otherwise be the case. In other Annelida, however, the primitive mesoderm- 

 cells are met with at the posterior edge of the blastopore (Vol. L, pp. 264 and 

 283), but the condition is different from that in Peripatus in so far as the 

 blastopore does not pass direct into the anus. In view of these possible differ- 

 ences between the species of Peripatus inter se and of the possible resemblances 

 to some Hirudinea, a renewed investigation of the relations of the growing zone 

 in P. novae-zcalaadiae, which must be regarded as the most primitive form [one 

 of the most specialised according to Willey and v. Kennel], is much to be 

 desired. 



The South American species, in conse- 

 quence of the small size of their eggs 

 and of the connection of the latter with 

 the wall of the uterus, have a different 

 form in the early stages. Our description 

 of the embryo ceased at a stage in which it 

 was somewhat pear-shaped (Figs. 80 and 81). 

 From this stage it passes into the mushroom 

 stage (Fig. 87), the embryo proper gaining 

 in size as compared with the umbilical cord, 

 through extending in both directions at right 

 angles to the axis of the cord (Fig. 87). 

 These two directions correspond to the length 

 and the breadth of the embryo. Growth 

 takes place at first chiefly in the first of these two directions, 



N 



Fio. S5. — Embryo of P. 

 capensis (after Balfour 

 and Sedgwick), a, anus; 



«!, mouth. 



