DEVELOPMENT OF PERIPATUS NOViE-ZEALANDI^. 269 



one half of its surface, at which time it is thickest in the centre 

 and thins gradually towards its edges. 



7. The epibolic growth of the blastoderm continues until 

 only a very small space in the middle of the ventral face of 

 the ovum is left uncovered. 



8. A proliferation of the nuclei behind the uncovered area 

 in the middle line takes place, forming a (in transverse section) 

 keel-shaped mass of nuclei extending along about the posterior 

 half of the ovum. The nuclei at the sides of the space also 

 proliferate. 



9. The protoplasm round the space becomes inflected, and so 

 forms a blastopore. The proliferating mass of nuclei or primi- 

 tive streak increases in amount. 



10. The blastopore increases in length considerably, and 

 becomes more open. The primitive streak also becomes wider 

 and deeper, and a groove — the primitive groove — appears along 

 its centre. Beneath the primitive groove a small cavity filled 

 with yolk and bounded by columnar nuclei, and apparently 

 homologous with the polar area of P. capensis, appears. 



11. The blastopore and primitive streak and groove increase 

 and become more marked. The polar area disappears. 



12. Up to this stage no trace of any cell-outlines is visible, 

 but the protoplasm forms a syncitium, in which nuclei are 

 irregularly scattered. At this point a large gap is present in 

 my investigations. 



13. A layer of yolk is present outside the embryo. This 

 peripheral yolk becomes gradually absorbed, and various 

 changes are undergone by the embryo (descriptions of which 

 are given in (3), vide summary, p. 255) until it reaches the 

 stage at which the absorption is complete, when the appendages 

 begin to appear, &c. 



14. The later development, i.e. after the appendages are 

 formed, is similar to that of P. capensis, the only interesting 

 point of difference being that the duct of the first somite opens 

 to the exterior. 



