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4. The Cleavage of the Ovum in Crepidula fornicata. 



By E. G. Conklin, Delaware, Ohio, U. S. A. 



eingeg. 7. März 1892. 



A preliminary note on the embryology of Crepidula was published 

 in the Johns Hopkins University Circulars No. 88. The present paper 

 sets forth a few facts which are not mentioned in that article or were 

 but briefly alluded to. It will be followed shortly by a more extended 

 paper on this subject. 



This work was conducted during two summers (1890 — 91) at the 

 laboratory of the TJ. S. Fish Commission at Wood's Holl, Mass., and 

 during the intervening winter in Prof. Brooks' laboratory at the Johns 

 Hopkins University. 



The adult Crepidula is fastened to a shell or stone or some other 

 object by its broad flat foot which is slightly concave and forms a po- 

 werful sucker. It never moves about upon this object but remains its 

 whole life long in the same position. If forcibly removed from its 

 support it is helpless and dies apparently without making an effort 

 to again attach itself. Young Crepidulas however move about quite 

 freely and if detached can easily attach themselves again. 



The eggs are laid in pouches which are fastened together and 

 attached to the object upon which the parent lives just in front of the 

 foot and under the shelter of the shell. Here they are aerated by 

 currents of water which are swept into and out of the branchial cham- 

 ber. About 50 eggs are laid in each pouch almost all of which devel- 

 ope normally, only a few developing into abnormal embryos , the so 

 called »cosmellae«. Fig. i. 



The method of determining the relation 

 of the first cleavage furrow to the axes of the 

 embryo is so far as I know a novel one and 

 I shall therefore describe it at some length. 

 After the appearance of the first furrow 

 the ovum when seen from the animal pole 

 appears as in Fig. 1 the cleavage furrow 

 being a straight line. As the second furrow 

 begins to appear the two blastomeres be- 

 come indented at their outer sides and at 

 the same time the first furrow is bent at 

 its middle toward the right, Fig. 2 (the 

 first furrowbeing in the line of vision). Then 

 as the second furrow approaches comple- 

 tion this bend in the middle of the first furrow increases and the two 



