-23- 



of the medial part (^Ig. 5 h, l) of the ciliated layer with the 

 adjacent side (l') of the deep, ciliated groove (v.c.g.). Refer- 

 ence line 2 shows where the lateral part (2) of the cover layer 

 joined the outer side (2') of the ciliated pouch (p.). Reference 

 line 3 indicates the point at which the lateral part (3) of the 

 cilia,ted layer joined the medial side (5') of the ciliated 

 pouch (p.). m.l. is the median lamella formed by the union of 

 the medial portions of the cover layers (d.l.) of the two parts 

 of the gland; from this point it is an incomplete partition all 

 the v/ay to the posterior end of the gland. 



Posterior to the duct (Fig. 5 j), the medial portion 

 (1) of the ciliated layer joins its fellow of the opposite side, 

 above the median lamella (m.l.), while the outer part (3) of this 

 layer remains continuous with the outer part of the cover layer. 

 By this arrangement the lateral invagination (i.v.) apparently 

 completely separates a lateral from the central part of the 

 gland, as has been mentioned before, and the mesoblast cells are 

 found between the two divisions (mes.). This is shown also in 

 Fig, 5 m, v/hioh is a horizontal section of the entire gland. 

 A section at this plane gives the impression that the lateral 

 invagination (i.v.) almost completely separates the lateral from 

 the central portion of the gland, but of course if the section 

 were cut nearer to the dorsal surface of the gland, the lateral 

 portion would be seen to be joined to the central portion ante- 

 rior to the duct (to the left of m.l.), as has been seen in 



