ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 151 



are to be looked upon as the terminal region of the brain, as a " telen- 

 cephalon " in the sense of His. 



In Lepidosiren, the author finds that the motor nerve can be traced 

 back to an extremely early stage of development, when it is found in the 

 form of a soft protoplasmic bridge connecting spinal cord and myotome, 

 which are still in contact. As development proceeds, the myotome 

 becomes pushed outwards by the development of mesenchyme, and the 

 nerve-trunk becomes correspondingly lengthened. The author also 

 discusses the development of the primitive nerve fibrils, the genetic 

 affinities of the lower Gnathostomata, the cellular constitution of the 

 Vertebrate body. The primitive cell-layers — ectoderm and endoderm — 

 may be thought of as consisting primarily of an epithelial layer of tailed 

 cells (as seen in Lepklosiren ectoderm). Parts of the primitive endoderm 

 become nipped off to form ccelomic lining (including the myocoelic 

 wall, which becomes partially converted into muscle), while individual 

 cells of both primary layers migrate into the cavity between them, and 

 give rise to the mesenchyme and its derivatives. In the nipped-off parts 

 of primitive endoderm which line the enterocoelic outgrowths, we may 

 see persisting representatives of the coelenteric pouches of the Scypho- 

 zoon coelenterate, and in the immigration of mesenchyme cells a con- 

 tinuation of the similar process so beautifully seen in the immigration of 

 the skeletogenous cells of Alcyonaria. 



Optic Chiasma of Teleosts.* — A. P. Larrabee has studied the 

 dimorphism in the relation of the optic nerves where they cross in 

 Teleosts. The right nerve (i.e. the nerve to the right eye) may be 

 either dorsal or ventral to the left. He dissected 4950 specimens of 

 Salveliiius, and found that 2749 had the right nerve dorsal, and 2201 

 the left nerve dorsal. He dissected 1132 specimens of Gadus, and found 

 that 580 had the right nerve dorsal, and 552 the left nerve. There is 

 thus a slight excess of right. The dimorphism is not inherited ; it is 

 not due to an earlier development of one of the nerves : gravity has no 

 effect on the nature of the crossing ; j\Iendelian principles do not apply ; 

 Galton's law is likewise inapphcable. The condition of the crossing is a 

 matter of chance. 



W. E. Castle adds in a note that it is somewhat remarkable that the 

 rights should uniformly predominate. The case may perhaps belong to 

 the same category as the more frequent occurrence of polydactylism in 

 guinea-pigs upon the left side of the body. 



Development of Tentacular Duct of Xenopus.t — L. Cohn, con- 

 tinuing his investigation of the tentacular apparatus of Xe/iopus cal- 

 caratus, finds that the tentacular duct, like the two tentacular ducts "in 

 Ichthyophis glutinosa, arises from an epithelial ridge constricted off from 

 the epidermis and sunk inwards. It is quite independent of the nasal 

 primordium, though it becomes secondarily connected with it. 



Origin of Mesenchyme in Sturgeon.^ — S. Tikhenko notes that 

 embryologists are divided into two camps in regard to the origin of the 



* Proc. Amer. Acad. Arts and Science, xlii. (1906) pp. 217-31. 

 + Zool. Anzeig, xxxi. (1906) pp. 45- 53 (7 figs.). 

 : Op. cit., XXX. (1906) pp. 728-30 (2 figs.). 



