DEVELOPMENT OF THE OAPE SPECIES OF PERIPATUS. 199 
causes a corresponding readjustment in the density of the 
network at different parts of the ovum, but no break in con- 
tinuity. 
The Gastrula arises by a process of epibole and is at first 
solid. 
The endoderm masses at first have no nuclei. Nuclei first 
appear in them during the progress of the epibole by which the 
gastrula is formed. I have not been able to determine the 
origin of these nuclei. They either arise de novo in the 
endoderm masses or migrate into the latter from the ectoderm. 
The protoplasmic network at the centre of each endoderm 
mass is denser than at the periphery, but is without the 
chromatin granules, so characteristic of a nucleus. But I have 
described a stage of the nucleus in the fertilised unsegmented 
ovum in which the chromatin granules are almost entirely 
absent, and in which the network presents no essential difference 
from the surrounding network. Again, another in which the 
nuclear network merges so gradually into the surrounding net- 
work, that it is impossible to point to any limit between them. 
I therefore think it quite possible that this central denser 
protoplasm in the endoderm masses may give rise to the nucleus 
which subsequently appears. 
The gastrula is a syncytium; the ectodermal nuclei are 
arranged around the periphery of the ovum, while the endo- 
dermal nuclei are within. The latter are characterised by 
their angular shape, and by never presenting the karyokinetic 
figures characteristic of the ectodermal nuclei. The protoplasm 
of this syncytium is much vacuolated throughout, but the 
vacuoles are largest in the centre. These central vacuoles 
unite and give rise to the gut cavity, which opens to the 
exterior through a point on the surface where the ectodermal 
nuclei have always been absent. This opening is the blastopore. 
The blastopore, until quite late in development, is traversed by 
protoplasmic strands, which anastomose with similar strands 
projecting from the protoplasm lining the large central vacuole 
or gut. 
The gut of Peripatus arises, therefore, as a vacuole in a 
