DEVELOPMENT OF THE CAPE SPECIES OF PERIPATUS. 185 
vacuoles in the centre of this mass. The gut of Peri- 
patus is therefore to be looked upon as a vacuole, resembling 
in all essential respects the cavity in the body of a ciliated 
Infusorian. I refer to Pl. XIII, fig. 20, which represents a 
section through a gastrula in which the gut is only just 
appearing, and to Pl. XIV, figs. 23, 24, which represent 
sections through a rather later stage, in which the gastrula 
cavity is established. In fig. 23 especially, the gut is seen to 
be traversed by a protoplasmic reticulum containing a nucleus, 
and the blastopore itself to be partially choked up by a similar 
reticulum. The latter feature is also seen in fig. 24 0, a 
section of a slightly older embryo, and, indeed, is characteristic 
of all the later gastrula stages until the definite division of the 
blastopore into the primitive mouth and anus. The gut 
vacuole, soon after its appearance, acquires an opening to the 
exterior through a point on the surface where the ectodermal 
nuclei are and always have been absent. 
Tue Various Forms or NucLeI IN THE HEARLY STAGES OF 
DEVELOPMENT. 
I have no observations on the nucleus of the ripe ovum. 
The facts which I have to record on the structure of the 
nucleus after the entrance of the spermatozoon may be 
described under the following heads: 
1. The nucleus of the unsegmented ovum after the conju- 
gation of the male and female pronuclei of the ectoderm ‘cells 
in the early stages of segmentation. 
2. The nucleus of the ovum before this event, but after the 
entrance of the spermatozoon. 
3. The nucleus of the ectoderm during the segmentation 
and gastrula stages. 
4. The endodermal nuclei. 
1. The nucleus of the completely fertilized ovum and its im- 
mediate descendants is so large and favorable for study that 
I have decided to describe it first. It varies considerably in 
shape avd structure in different ova. These variations no 
