THE FORMATION OF THE MEDULLARY GROOVE 

 AND SOME OTHER FEATURES OF EMBRYONIC 

 DEVELOPMENT IN THE ELASMOBRANCHS.i 



WILLIAM A. LOCY. 



In studying the mode of formation of the body of Elasmo- 

 branch embryos,^ I have noticed some points regarding the 

 formation of the medullary groove, the construction of the 

 cephalic plate, and the early formation of the eyes, that seem 

 to me of sufficient interest to justify a preliminary communica- 

 tion, inasmuch as the publication of my memoir dealing with 

 these and other topics of Elasmobranch development will be 

 considerably delayed. 



The early formed groove which appears in very young em- 

 bryos of Elasmobranchs — in Balfour's Stage B — has long 

 been known, and the method of its formation has been many 

 times correctly described (Balfour, His, and subsequent writers). 

 This groove is conventionally stated to be formed by the " Con- 

 crescence of the embryonic rim." It is consistently referred 

 to in Balfour's various publications as the medullary groove, 

 and he has been followed in this respect by other writers, 

 including the Zieglers^ in 1892. 



A careful comparison of sections arranged in close sequence 

 of development has led me to the conclusion that the early 

 formed median furrow — or "Concrescence furrow," as it might 

 be called — is not the medullary groove.* It occupies the same 



1 Read before the American Morphological Society at Princeton, New Jersey, 

 December 28, 1892. 



2 This line of study was taken up at the suggestion of Dr. C. O. Whitman, and 

 was carried on at the Wood's Holl Biological Observatory during the summer of 

 1892. 



8 Archiv flir Mikroscopische Anatomie, Bd. 39, S. 56-98. 



* Kastschenko expresses substantially the same conclusion when he says : " Die 

 Primitivwiilste sind mit den Medullarwiilsten nicht identisch " etc., (Anat. Anz., 

 Bd. Ill, 188S, s. 455); and Adam Sedgwick in his article of last June refers, in a 

 passing way, to the early fissure as a transitory structure (Q. J. M. S., June, 

 1893). 



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