No. I.] THE EMBRYOLOGY OF CREPIDULA. 1 43 



which are formed on the ventral side anterior to the growing- 

 point probably correspond to the ventral plate of Unio, and 

 also, as in that animal, give rise to the foot. There is good 

 reason to believe that these cell rows ultimately come from 

 the cell 2d. 



The cell origin of the foot is not given by either Blochmann 

 or Heymons. From the latter's figures, however, it is evident 

 that its place of origin and early history in Umbrella is essen- 

 tially the same as in Unio and Crepidula. 



In Patella (Patten, '86) and Fulgur (McMurrich, '86) the foot 

 is said to arise as two lateral swellings which subsequently fuse 

 together on the ventral mid line. Although it is single in its 

 origin in Crepidula, the row of large transparent cells along its 

 median surface gives it the appearance of being double, espe- 

 cially in the large embryos of C. convexa and C. adunca ; how- 

 ever, careful study of profile views and of sections shows that it 

 is not double, but is a single median protuberance. 



(f) The External Excretory Cells. — On each side of the 

 embryo, just posterior to the velum and dorsal to the foot, 

 several of the ectoderm cells swell up and gradually lose their 

 nuclei and cell boundaries. Fig. 78 ; the cells become vacuo- 

 lated, and the vacuoles are filled with small granules which 

 stain deeply. Later the several vacuoles seem to flow together 

 into one or more large ones. Figs. 80, 81. In the early stages 

 these cells form a part of the ectodermic layer, but as the 

 embryo grows older they grow more prominent, and the whole 

 mass is constricted at the base, so that it becomes pear-shaped, 

 the narrower end being attached to the embryo and the larger 

 end being distal. Figs. 81, 104. The surrounding ectoderm 

 cells crowd in at the neck of this constriction, and work their 

 way entirely beneath these excretory cells. About the begin- 

 ning of the free larval life many of the vacuoles with their 

 granular contents disappear, and there is left on each side a 

 clear, pear-shaped mass, which is attached for a time in the 

 deep constriction posterior to the velum. Ultimately these 

 structures appear to be pinched off completely. I have not 

 observed them in the process of being cast off, but they sud- 

 denly disappear and leave no trace behind, except that one 



