180 



CHAS. B. WILSON. 



This cavity contains one or more loose cells whicli have 

 been given off from the entoderm cells where they join the 

 ectoderm. The loose cells are mesenchyme, and so far as 

 observed none of them originate from the ectoderm. 



Gastrula. — As soon as the cells begin to multiply for 

 invagination and the formation of the intestine they become 

 equal in 'size^ and their inner ends are evened. The mesen- 

 chyme still continues to come off from the entoderm, and the 

 latter is now clearly differentiated from the ectoderm (fig. 69). 



The ectoderm consists of a single layer of epithelium. 

 On the aboral surface it is a pavement epithelium made up 

 of cubical cells, which are small at the superior pole, grow 

 larger toward the equator, and then grow smaller near the 

 border of the oral surface. By a comparison of different 

 stages we find that the cells around the apical plate are the 

 first to become transparent (fig. 70). Large clear spaces 

 like vacuoles appear in these cells ; but they are not empty 

 spaces, for they react to stains in the same way as the 

 gelatine which fills the body-cavity. These spaces gradually 

 extend down toward the oral surface, until by the time the 

 lappets are formed they can be found in every cell of the 

 aboral ectoderm (fig. 73). In individual cells they are at first 

 spherical, but quickly elongate parallel with the ectoderm 

 surface. Then together with the cells they increase in area 

 at the expense of their thickness, leaving the nuclei em- 

 bedded in cytoplasm on the inner border of the cells. 



The oral ectoderm is much thicker, and composed of 

 cylindrical cells in which no clear spaces can be seen. These 

 cells have larger nuclei than those in the aboral ectoderm, 

 and are more active. From them are developed the lappets, 

 the cilia row which surrounds the edge of both oral surface 

 and lappets, and the invaginations which form the amnion 

 and the ectoderm of the adult Nemertean (13). 



The entoderm is much thicker than the ectoderm, but 

 consists of a single layer of cells, cylindrical in form and 

 similar to those of the oral ectoderm. They are more 

 irregular in shape and overlap somewhat, so that they give 



