No. I.] THE EMBRYOLOGY OF CREPIDULA. 93 



observations on Umbrella, and my own results on four species 

 of Crepidula. 



Kofoid ('94) also has expressed the view that Blochmann was 

 wrong in the derivation of the basal cells, and as he presents 

 other evidence in favor of the position here taken, I quote his 

 words: "There are indications, however, that they (the basal 

 cells) were really derived from the apical quartette ai-di; for 

 (i) their nuclei are nearer those of the apical quartette; (2) the 

 cells of the apical quartette are much smaller after the cells 

 a"2-d"2 appear than before ; (3) a'2-d'2 have just arisen by a 

 recent division, whereas some time has elapsed since the first 

 division of the apical quartette." 



The subsequent division of the basal cells is identical with 

 their division in Crepidula, not only in the direction of the 

 cleavage and the size of the resulting cells, but also in the time 

 of its occurrence. This is the more remarkable when it is con- 

 sidered, as will be done in a moment, that in both Crepidula 

 and Neritina the direction of this cleavage really detenni^ies the 

 continued existence of the ci'oss, and further, that it violates the 

 "law of alternating cleavages!' 



In its later history the identity of the cross in Neritina and 

 Crepidula is still further emphasized. At a stage with 49 

 ectoderm cells (Blochmann's Fig. 56, corresponding approxi. 

 mately to my Fig. 36 or 38), the cells of the posterior arm of 

 the cross divide so as to form a series of four cells, while each of 

 the other arms contains but three. This is such a remarkable 

 agreement with what takes place in Crepidula, that I think it 

 worth while to quote Blochmann's words on this point (p. 158) : 

 " Besonders bemerkenswerth erscheint bei diesem Stadium das 

 Auftreten einer vierten Zelle in der aus drei Zellen bestehenden 

 Reihe, die in der Mitte der hinteren Halfte der Ektoderm- 

 scheibe verlauft, wahrend in den drei anderen entsprechenden 

 Zellreihen die Dreizahl erhalten bleibt. Das Auftreten dieser 

 Zelle ist ein ganz Konstantes und wurde an fiinf Praparaten 

 beobachtet. Man kann wohl sagen, dass in der Ektoderman- 

 lage an und fur sich erst durch das Auftreten dieser Zelle vorn 

 und hinten unterscheidbar wird, wahrend vorher nur die Rich- 

 tung der Sagittalachse durch das Vorhandensein der Kornchen- 



