1904.] NATURAL SCIENCES OF PHILADELPHIA. 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’, 2’, 
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 with 
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 leotropic division. This cell soon cleaves dexiotropically 
into two of equal size. At the next cleavage there result in Crepidula 
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't, 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, M', M?; 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- 
