No. I.] THE EMBRYOLOGY OF CREPIDULA. 73 



of 4d up to a very late stage, and with such accuracy of detail 

 that one can scarcely doubt the soundness of his conclusions. 

 In fact, his results have led me seriously to doubt the correct- 

 ness of my own interpretations ; but after a careful reexamina- 

 tion of my preparations and drawings, I am unable to reach any 

 other conclusion than that already given. I cannot say that 

 I have actually observed the cells which I have called the 

 enteroblasts going into the formation of the intestine. The 

 last stage in which they could be identified beyond any doubt 

 is shown in Fig. 65, and between this stage and those shown 

 in Figs. 6Z and 'j6 there is more or less discontinuity. Still I 

 believe that the evidence is clearly in favor of the view which 

 I have advanced. The enteroblasts Ei,E2,ei,e2 are not only 

 in exactly the right position to form the distal end of the intes- 

 tine, but they differ in histological character from the meso- 

 blast cells, and closely resemble the entoblasts in that they 

 contain a number of yolk spherules ; besides in all the later 

 stages the large cells which I have called the mesoblastic telo- 

 blasts, M' and M^, are plainly visible lying at the posterior ends 

 of the rather indistinct mesoblastic bands. These cells are fre- 

 quently seen dividing, but during all the time that the mesoblastic 

 bands are forming, the enteroblasts never divide, which is, I 

 think, pretty good evidence that they are not the mesoblastic 

 teloblasts. 



In Heymons' figures of the later stages (see his Figs. 29 

 and 30), the position and appearance of the mesoblastic telo- 

 blasts is so similar to the corresponding cells in Crepidula, 

 that one can scarcely doubt that they are homologous cells. 

 However, between the two teloblasts in Umbrella is a group of 

 small cells, which, from their position, should correspond to 

 the enteroblasts of Crepidula, but which seem to be very dif- 

 ferent in origin and history from these cells. Of this group 

 of cells Heymons says, p. 281 : " Zwischen den beiden aus 

 einander gewichenen Urmesodermzellen befindet sich eine 

 Anzahl von 4-6 kleineren Mesodermzellen. Dieselben ent- 

 sprechen der Zellgruppe, an deren Bildung die zuerst entstan- 

 denen kleinen Mesodermzellen m m Antheil nahmen. Letztere 

 sind jetzt allerdings nicht mehr als solche herauszufinden, 



