SUMMAET. 43 



The gut of Peripatus arises^ therefore, as a vacuole in a 

 multinucleated mass of protoplasm, and the gastrula of 

 Peripatus is a multinucleated mass or syncytium, with absolute 

 continuity of the protoplasm of all parts of the ovum. 



The Mesoderm. — After the definite formation of the blastopore, 

 an area of protoplasm, placed in the ectodermal layer of the 

 syncytium, and characterised 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. 

 This area I have called the polar area. Its nuclei undergo 

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

 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. 



The nuclei of the primitive streak migrate forwards between 

 the ectodermal and endodermal nuclei, and take up their po- 

 sition in the protoplasm intervening between the latter. 



These rows of nuclei are the mesodermal bands. They soon 

 arrange themselves into groups around 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 commence- 

 ment of the formation of the somites, merely stating that it is 

 still a syncytium. 



There are a certain number of facts in the above account 

 which are of general interest and seem to deserve more dis- 

 cussion so far as their relation to processes in other forms are 

 concerned. These are : 



1. The connection between the intra- and extra-nuclear re- 

 ticulum. 



2. The segmentation. 



3. The origin of the gut as a vacuole. 



4. The syncytial nature of the embryo. 



5. The origin of the mesoderm. 



I propose to consider the first four of these points at once, 

 and to defer the discussion of the 5th to the next chapter. 



A. The nucleus of the unsegmented ovum and of the early 

 stages of segmentation of the Cape Peripatus are particularly 



