342 PROCEEDINGS OF THE ACADEMY OF [April, 



relatively small, the same result being here obtained as in Fiona, in 

 which the entoblastic elements are produced from the anterior rather 

 than from the posterior side of the teloblasts. If any of the descend- 

 ants of 4d of Umbrella described b}- Heymons are entoblastic in nature 

 they are those which arise in this way, and these are the cells which 

 must be compared with the enteroblasts of Crepidula and the small 

 anterior cells in Fiona. 



Viguier (1898) describes and figures the formation of the mesoderm 

 in Tethys fimhriata as similar to that of Umbrella, and a comparison of 

 figures will show almost exact correspondence. Like Heymons, 

 Viguier does not consider the derivatives of 4d to be other than meso- 

 dermal in fate. 



Carazzi (1900) derives both mesoderm and endoderm from the 

 cell 4d ("EM") of Aplysia. He states that the cleavage w^hich forms 

 this cell is dexiotropic in direction, and such appears to be the case 

 from his figures. The cell 3A of Aplysia is larger than the others, 

 thus throwing 3D so much to the right of the median line that a dexio- 

 tropic cleavage is necessary to place the mesentomere upon this line. 

 The divisions of 4d which follow are identical with those of Fiona, but 

 Carazzi's conclusions regarding the fate of the remaining blastomeres 

 are quite different. Four pairs of small cells are derived from the two 

 large cells and lie anterior to them. These correspond in position to 

 the four (or more?) enteroblasts of Fiona, but by Carazzi are described 

 as mesodermal. Two larger cells have been given off posteriorly and 

 correspond to m*z\ m^z^ of Fiona. From each of these a small cell 

 buds forth posteriorly, the two lying near the ectoderm. These small 

 cells are, according to Carazzi, enteroblasts, and go into the intestine. 

 Cells similar to these in origin and, for the time at least, in position are 

 found in Fiona (z\ z^) lying closely pressed against the ectoderm in 

 the posterior region of the gastrula. They are small in size, and at a 

 later time I have found it impossible to distinguish them from many 

 small mesodermal cells which crowd that region of the gastrula. If 

 they do not shift their position, they would naturally become involved 

 in the formation of the distal end of the intestine either directly, as 

 hning cells of that organ, or as muscle cells for its walls. One cannot 

 help feeling in comparing the development of the two forms and noting 

 the great similarity in the history of the early derivatives of 4d that 

 their fate is also the same ; and the same might also be said of the small 

 anterior elements which Carazzi indicates as mesodermal. 



Lillie (1895) concluded that in Unio the derivatives of 4d were 

 entirely mesoblastic. The two teloblasts give origin to two small cells 



