1885.] 



Ovum of the South African Peripatus. 



243 



arranged round the periphery of the ovum, while the endodermal 

 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 multi- 

 nucleated mass of protoplasm, and the gastrula of Peripatus is a multi- 

 nucleated mass or syncytium, with absolute continuity of the proto- 

 plasm of all parts of the ovum.. 



The Origin of the Mesoderm. 



After the definite formation of the blastopore, an area of proto- 

 plasm, placed in the ectodermal layer of the syncytium, and charac- 

 terised by possessing several nuclei less densely packed together than 

 elsewhere, is distinctly visible in the middle line of the ventral surface 

 just behind the blastopore. I cannot be certain of the exact number 

 of the nuclei belonging to this area in the youngest embryo in 

 which I observed it, as the limits of the area are difficult to 

 determine- by inspection of transverse sections. However that may 

 be, the- area has in transverse section very much the appearance of the 

 pole cells of other forms, and is the structure described in my first 

 paper as " some cells which cannot be definitely assigned to the 

 ectoderm or to the endoderm, at the hind end of the blastopore." Its 

 nuclei undergo division and give rise to the densely packed mass of 

 nuclei of the primitive streak.* A part of it seems to persist for 

 some time in the deeper parts of the primitive streak close to the 

 endoderm : I have not yet succeeded in tracing the fate of this 

 portion. 



The nuclei of the primitive streak migrate forwards between the 

 ectodermal and endodermal nuclei, and take up their position in the 

 protoplasm intervening between the latter. 



These rows of nuclei are the mesodermal bands. They soon arrange 

 themselves into groups a.round a central vacuole, and so give rise to 

 the most conspicuous parts of the mesoblastic somites. I leave the 

 ovum for the present at the commencement of the formation of the 

 somites, merely stating that it is still a syncytium. 



* It is possible that some of the ectoderm nuclei adjoining this area may take 

 part in the production ot the primitive streak nuclei. 



