CLEAVAGE AND FORMATION OK THE GERM-LAYKKS. 



89 



J3. 



pole of this blastula is therefore easily distinguished from the vege- 

 tative pole; the latter soon heroines somewhat flattened, its cells 

 becoming invaginated to form the archenteron (B). Dentalium has 

 thus a typical in- 

 vagination- gastrula pi 

 (C) whose trans- 

 verse axis is some- 

 what broader than 

 its invagination- 

 axis. A few large 

 blast omeres soon 

 become detached 

 from the outer 

 surface and pass 

 into the cleavage- 

 cavity (Fig. 34 C). 

 Here they become 

 arranged with bi- 

 lateral symmetry 

 and are found, 

 especially at later 

 stages, near the 

 blastopore. They 

 represent the rudi- 

 ment of the mesoderm {mes). This latter, which at first consists 

 of only a few large cells, soon takes the form of two groups of 

 cells which, after increasing still further in number, form the two 

 mesoderm-hands which lie near the archenteron. 



Fig. B4..—A-C, sections through embryos of Dentalium in 

 the blastula and gastrula-stages (after Kowalevsky). 

 hi, blastopore ; mes, rudiment of the mesoderm : w, cells 

 of the ciliated ring; ws, ciliated tuft. 



The mesoderm-rudiment, as has been mentioned, shows a bilateral 

 symmetry, but this is not so regular as, for instance, in Chiton (p. 4). In 

 >uiik\ indeed, of Kowaleysky's figures the bilateral symmetry is distinct, but 

 in others it appears to be less regular. This also applies to the mesoderm in 

 its later development. A cavity does, indeed, appear in the mesoderm which 

 Kowalevsky is inclined to regard as the coelom, but the stage in which it 

 appears is a comparatively late stage, the body being already somewhat de- 

 veloped. These points are, in fact, not sufficiently well understood to justify 

 us in drawing any definite conclusions. 



In connection with the formation of the mesoderm, it should be mentioned 

 further that, at the blastulastage, i.e., when invagination is commencing, 

 isolated cells of various sizes are to be met with in the cleavage-cavity : these 

 maj possibly be mesoderm-cells, although Kowalevsky himself seems to be 

 inclined t>> think that the mesoderm arose in the way above described, and to 

 consider the occurrence of the^c cells in the cleavage-cavity as abnormal. 



