EXPLANATION OP PLATES III, IV, & V, 



List of Reference Letters. 



p. b.y First polar body. p. b.^. Second polar body. f. n. Female pronu- 

 cleus, m. n. Male pronucleus, o. c. Cavity in centre of ovum. s. b. More 

 deeply-staining bodies in extra-nuclear part of ovum. ec. Ectoderm, en. En- 

 doderm. e. s. Egg-shell, n. Network, tt. e. Uterine epithelium, bl. Blas- 

 topore, g. g. Gut of gastrula. p. a. Polar area. p. g. Primitive groove. 

 p. si. Primitive streak, m. b. Mesoblastic band. 



Fig. 1. — Section through the fertilised ovum of Peripatus Balfouri 

 before the conjugation of the male and female pronuclei. The female pro- 

 nucleus is at the periphery of the ovum. Its reticulum is very loose. Large 

 masses of deeply-staining matter are present. The extra-nuclear reticulum is 

 not denser round either the male or female pronuclei than elsewhere. A large 

 cavity is present in the centre of the ovum. The extra-nuclear reticulum is 

 only drawn in immediately round the two nuclei. Elsewhere it is only indi- 

 cated by shading. It is completely absent in the centre of the ovum. Peculiar 

 bodies of irregular shape, staining more deeply and continuous by means of 

 processes with the reticulum, are present. They are probably merely expan- 

 sions of the strands of the reticlum. The male pronucleus is on the opposite 

 side of the ovum to the female, but rather nearer the centre. It does not, 

 however, lie in the same transverse plane as the female nucleus, though very 

 nearly so. It is formed of a network, precisely similar in character to the 

 extra-nuclear reticulum. The membrane round the nucleus is continuous 

 with both the extra- and intra-nuclear reticulum. Deeply-staining bodies 

 are present in the intra-nuclear parts of the network. Near the female 

 pronucleus is the second polar body, with a small portion of the first attached 

 to it. Greatest diameter of female nucleus "029 mm. The male nucleus 

 measured '025 x "016 mm. Drawn with Zeiss's camera, f, oc. 2. Picric 

 acid. f. n. Female pronucleus. m. n. Male pronucleus, oc. Cavity in 

 centre of ovum. p. b.^. First polar body. p. b.^. Second polar body. s. b. 

 More deeply-staining bodies in extra-nuclear part of ovum. 



Fig. 2. — Nucleus of unsegmented ovum of Peripatus capensis in 

 spherical stage. Network more diffusely stained than in Fig. 8. Borax car- 

 mine. Drawn to same scale as Fig. 8. 



Fig. 3. — Nucleus of an ovum with two segments of Peripatus capensis. 

 Nucleus divided up into compartments by specially well-marked portions of 

 the nuclear network. The deeply-staining irregularly-shaped masses are 

 almost certainly contained in the strand of the network. The nuclear net- 

 work is most distinctly continuous with the extra-nuclear reticulum. Reticu- 



