DEVELOPMENT OP PERIPATUS NOV^-ZEALANDIiE. 265 



about one third of the length of the ovum, and having in 

 transverse section an irregular triangular shape, the base of 

 the triangle resting on the surface; nuclei are also present 

 throughout the yolk, more especially near the periphery. 



As was said in (3) the protoplasmic area next becomes more 

 compact and flattened out against the side of the ovum, so as 

 to form a kind of plate of fairly dense protoplasm which is 

 closely packed with nuclei, and which lies on the surface at one 

 side of the ovum through about the middle third of its length : 

 this is shown in transverse section in fig. 13 (3), which is from 

 one of the ova taken out in April. 



A good many of the January eggs were of stages which 

 came after this last. The protoplasmic area begins to grow 

 round the surface of the egg until, as is shown in fig. 1, it 

 covers nearly half the periphery. The nuclei in the central 

 region of this protoplasmic area are two or three deep, but 

 towards its edges the protoplasm thins gradually and the nuclei 

 form only one layer. The ovum is closely packed with yolk, 

 among which a few nuclei are irregularly scattered. The pro- 

 cess of epibolic growth of the protoplasmic area continues, so 

 that, as is shown in fig. 2, still more of the periphery of the 

 egg becomes covered with the thin layer of protoplasm which 

 may be called the blastoderm, until when the growth of the 

 latter ceases only a very small portion is left uncovered. 



The blastoderm now consists of a thin layer of protoplasm, 

 in which a single layer of flat nuclei is present, extending 

 round the whole periphery of the egg with the exception of a 

 small space lying in the centre of one side of the egg in which 

 the yolk is left uncovered. This uncovered space corresponds 

 with the position of the future blastopore, and is, I believe, 

 situated on the ventral side of the embryo, thus agreeiug with 

 other Arthropods, except the Scorpions, in which the blasto- 

 pore is situated dorsally. 



A change now takes place in the blastoderm behind and at 

 the sides of the uncovered area. In the middle line behind 

 this area a proliferation of the nuclei takes place extending 

 some distance backwards, so that a keel-shaped mass of nuclei 



