1904.] NATURAL SCIENCES OF PHILADELPHL'^. 339 



toward the sides of the gastrula these small cells, which have been 

 derived from 4d, lie nearer the ventral surface than the cells which 

 form the bottom of the invaginating enteron and closely appressed 

 against the posterior boundary of this region. The small cells z\ z^, 

 which are the posterior derivatives of the division of m^z^, m^z^, also 

 continue to lie near the median line in the posterior region of the 

 gastrula, closely pressed and flattened against the ectoderm. 



The later history of the enteroblasts, which I believe are concerned 

 in the formation of the intestine, will be discussed in connection w^th 

 the development of the enteron. 



In comparing the mesoblast formation of Fiona with that of other 

 forms, Crepidula will be considered first, since in this Prosobranch 

 4d was first found to contain both entoblastic and mesoblastic material 

 (Conklin, 1897). Here 4d arises when twenty-four cells are present 

 and by a Isotropic division. This cell soon cleaves dexiotropically 

 into two of equal size. At the next cleavage there result in Crepidida 

 four cells of similar size, the posterior and lower pair being the first 

 enteroblasts, while in Fiona it is the anterior smaller cells which are 

 entoblastic. At the next cleavage in Crepidula the large cells Me^, Me^, 

 which still contain both mesoblast and entoblast, give off smaller 

 purely mesoblastic cells anteriorly (m^, m'), while in Fiona the larger 

 posterior cells give rise posteriorly to similar cells, though they may 

 not be purely mesoblastic. The next cleavage of M^e\ M^e^ in Cre- 

 pidula completely segregates mesoblast and entoblast, the cells of 

 the latter lying posterior to the mesodermal elements. This division 

 separates two more small enteroblasts in Fiona, which here lie with 

 the first enteroblasts anterior to the large cells, IVP, jVP; each gives 

 rise to another small cell anteriorly in Fiona which may be entero- 

 blastic, otherwise from this period on they function as teloblasts of 

 the mesoderm. 



From the above comparison it is evident that if we consider the 

 position of the mesodermal and endodermal constituents of 4d in 

 connection with the segmented egg as a whole, directly opposite 

 conditions are found. In Crepidula the derivatives of this cell form 

 mesoderm anteriorly and laterally, entoderm posteriorly, while in 

 Fiona the reverse is the case. But in both forms, if we consider the 

 position of the enteroblasts not in relation to the egg as a whole, but 

 only in connection with the macromeres with which they are to be 

 associated, it will be seen that in both Crepidula and Fiona these cells 

 are directed toward the posterior region of the cells 4D, 4C, or their de- 

 rivatives, and that the reverse relations of the enteroblasts and meso- 



