86 CASWELL GRAVE 



The definitive ganglion, so called because of its persistence as 

 the nerve center of the ascidiozooid, lies immediately above the 

 middle portion of the hypophysial duct (fig. 6). These struc- 

 tures are in close contact, but are not at this stage connected. 

 The ganglion is oval in form and is composed of a cortex of cells 

 and a medulla in which no nuclei are found (figs. 3 and 6) . The 

 nuclei of the cortex are small and do not have the structure 

 chacteristic of nerve cells, possibly because their functional 

 activity does not begin during the larval period. 



The subneural gland has the appearance of an enlargement 

 or outgrowth of the middle portion of the hypophysial duct on 

 the side opposite the definitive ganglion. It is a hollow struc- 

 ture and its cavity is in open communication with the lumen of 

 the hypophysial duct (figs. C and D). 



THE MIDDLE GERM LAYER 



Mesenchyme cells form a discontinuous layer just beneath the 

 body wall. They are found very infrequently in the posterior 

 part of the body, especially in the region of the sensory vesicle, 

 but in the anterior part they are quite numerous and, in the 

 parts from which the test vesicle and adhesive papillae have 

 developed, they form a continuous layer more than one cell in 

 thickness (fig. 9). Their distribution in the middle portion of 

 the body is shown in figure 3. At no place could they be said 

 to form an epithelium, and nothing comparable to the mesoderm 

 or coelom as they are developed in vertebrates is present. 



Mesenchyme cells of at least three varieties can be distin- 

 guished; one in which the cytoplasm is apparently homogeneous 

 is the most common, but another in which the cell bodies are 

 loaded with granules is not infrequent. The third variety is asso- 

 ciated with non-striated muscle fibers, of which some encircle 

 the body obliquely from the dorsal to the ventral side and are 

 located just beneath the mantle, some are distributed along 

 the walls of the pharynx and atrium, and quite numerous sets 

 of fibers extend in a radial direction from the oral and atrial 

 siphons as centers. 



