No. I.] THE EMBRYOLOGY OF CREPIDULA. 31 



These abnormalities may appear in the early stages of 

 cleavage, or they may be found in any of the later stages of de- 

 velopment, even up to the fully formed veliger. When present 

 in the early stages the blastomeres are more spherical and less 

 compact than usual. The four macromeres are frequently 

 separated from each other far enough to leave a cavity between 

 them, and into the depression thus formed the overlying ecto- 

 derm cells dip down, forming a pit. A similar invagination has 

 been described by Blochmann ('81) for Neritina and by McMur- 

 rich ('86) for Fulgur. Both of these authors supposed that 

 this was a normal feature of development, but from the loose 

 character of the cell aggregate and the rounded outlines of each 

 of the cells in Blochmann's figures of Neritina, I believe that 

 the ova there described were segmenting abnormally, and this 

 view is rendered all the more probable when the large propor- 

 tion (eighty to one) of abnormal as compared with normal eggs 

 in Neritina is taken into account. In fact. Figs. 52-56 of 

 Blochmann's paper represent very well the abnormally seg- 

 menting eggs of Crepidula, and I believe the ectodermal pit is 

 in Neritina, as in Crepidula, an abnormal formation. 



I have studied the cleavage in Fulgur and find that the ecto- 

 dermal invagination which McMurrich describes is the shell 

 gland which appears at an early stage, some distance posterior 

 to the apical pole ; it is therefore a wholly different feature 

 from the invagination in Neritina, which lies exactly at the 

 apical pole. 



Heymons ('93) found that in Umbrella, one of the Opistho- 

 branchiata, the egg capsules contain thirty to forty eggs, and 

 that some of these, though evidently no definite number, do 

 not develop normally. He says : "Von letzeren kommen 

 nicht alle zur normal Entwicklung, indem ein Theil von ihnen 

 gleich nach der ersten Stadien abweichende Verhaltnisse und 

 Missbildungen zeigt und spater dem Zerfalle unterliegt. Nicht 

 selten sind auch Doppel- oder gar Mehrfachbildungen zu be- 

 obachten, die durch Aneinanderwachsen der Eier zu Stande 

 kommen, wie sich im zwei- oder vierzelligen Stadium leicht 

 nachweisen lasst, und die demgemass auch die doppelte resp. 

 mehrfache Grosse besitzen. Solche Doppelbildungen sind haufig 



