482 ADAM SEDGWICK. 



form the linings of the vesicles, which are attached throughout 

 life to the veutral surface of the cerebral ganglia. 



3. The Stomodseal and Proctodseal 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 

 (PL XXXIV, figs. 5 Z>, 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 (PI. XXXIV, 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 

 (PL XXXIV, fig. 4, and PL XXXVI, fig. 28). It is by the 

 growth of this tissue that the lining of the stomodaeum and 

 proctodseum 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 (PL XXXV, fig. 23 d, s.gl). 

 They gradually increase in length and project into the central 

 compartment of the body cavity. Their further development 

 will be described more conveniently in Part IV of this series. 

 Kennel's description of the origin of these organs agrees with 

 my observations. 



