64 THE CHANGES FROM STAGE A TO P. 



by Kennel the ventral organs. The ventral organs are at 

 first placed at a little distance from the middle line. Eventually, 

 however, they approach one another and meet, excepting in the 

 region of the mouth and pre oral lobes. 



On the pre-oral lobes the structures corresponding to the 

 ventral organs of the posterior part of the body become invagi- 

 nated and separated from the surface in such a manner as to 

 form the linings of the vesicles, which are attached throughout 

 life to the ventral surface of the cerebral ganglia. 



3. The Stomodseal and Proctodaeal Ingrowths. — At the time 

 when the blastopore is a continuous slit and traversed by strands 

 of anastomosing protoplasm, i. e. during Stage a, the part of the 

 endoderm which is continued into the ectoderm at the lips of 

 the blastopore, resembles, in the small size and number of the 

 vacuoles and the regular shape and arrangement of the 

 nuclei, the ectoderm. In fact it is impossible to say whether 

 this layer is really ectodermal or endodermal. By its position and 

 development it resembles endoderm, by its characters ectoderm. 



When the blastopore closes in the middle these cells are left 

 inside and form the median ventral wall of the mesenteron 

 (Plate VI, figs. 5 b, 6 c, v. en.), and eventually assume the 

 characters of endodermal nuclei. At the primitive mouth and 

 anus, however, they still persist as rows of nuclei intervening 

 between undoubted ectoderm and endoderm (Plate VI, fig. 

 6 b, d), and they extend forwards for a short distance, forming 

 the median ventral wall of a portion of the pre-oral enteron 

 (Plate VI, fig. 4, and Plate VIII, fig. 28). It is by the growth 

 of this tissue that the lining of the stomodaeum and procto- 

 dseum is formed. 



The details of the development of these structures will be 

 best treated in the next section dealing with the alimentary 

 canal. 



4. The slime glands arise in Stage e as hollow invaginations of 

 the ectoderm of the oral papillae (Plate VII, fig. 23 d, s. gl.). 

 They gradually increase in length and project into the central 

 compartment of the body cavity. Kennel's description of the 

 origin of these organs agrees with my observations. 



