( 395 ) 

 NOTES AND MEMORANDA. 



ZOOLOGY. 



A. GENERAL, including Embryology and Histology of the 



Vertebrata. 



Primitive Stripe in the Chick.— The so-called primitive stripe, 

 or groove, had a future ascribed to it by the earlier embryologists, 

 which the more modern authors regard as belonging to a different set of 

 embryonic structures : to the elucidation of this question, Dr. Mathias 

 Duval has lately * devoted himself. Messrs. Foster and Balfour, in 

 their ' Elements of Embryology,' remark that " by the earlier observers 

 the primitive groove was supposed to become converted into the 

 medullary canal. Dursy (' Der Primitivstreif des Hiihnchens ') was 

 the first to give a correct account of its disappearance ; and the dis- 

 tinction between it and the medullary groove has since been fully 

 recognized by many observers." In addition to information which 

 confirms this view of Dursy 's, M. Duval describes the characters of the 

 adjoining regions. 



Bearing in mind the difficulty as to the exact stage at which an 

 embryo may be found to have arrived, a difficulty which is not lessened 

 but rather increased by information as to the exact length of the incuba- 

 tion period, M. Duval has not contented himself with merely stating 

 the number of hours. Imagine two embryos found at exactly the same 

 stage of development, but that one has been seventeen and the other 

 twenty-one hours in incubation. It is obvious that the ready method, 

 ordinarily adopted, of regarding the one as being some stages behind 

 the other, would easily lead to a belief in the succession of certain 

 changes, such as that which was asserted to obtain with regard to the 

 primitive groove. Another method of examination had therefore to be 

 adopted. Having obtained a series of blastoderms, of which he had 

 registered the length of the incubation period, M. Duval examined 

 them by surface view, and chose them out by pairs ; of each jjair one 

 remained intact and easily adapted to inspection, while the other was 

 hardened and stained, — and cut into sections ; these sections, again, 

 were not registered as being cut from an embryo of a definite age, 

 but from one (A, B, and so on) of which he had a corresponding and 

 intact example. 



The developmental period which was studied, was found to be 

 divisible into three groups; one was from the fom'teenth to the 

 twenty -second hour of incubation ; the second from the twenty-second 

 to the thii-ty-second ; and the third from the thirty-second to the 

 fiftieth hour. 



In the first period, inspection reveals the presence of an embryonic 

 " spot," which may be divided into two regions ; to the more anterior 

 and smaller i^art the name of tergal zone is given, while the rest forms 

 the immitive line, and, later on, the inimitive groove ; the zone grows 

 but little during this period ; in it the cjiiblast is alone well defined 

 * ' Ann. Sci. Nat. (Zoo!.),' vii. (1878) Nos. 5-6. 



