1889.] On the Early Development of Lepidosteus osseus. 



115 



in Elasmobranchii, the pronephros begins at the third definite body- 

 soniite, or, connting the three hypoglossus somites, at the sixth. Its 

 extension backwards varies, bnt the region of three somites is always 

 concerned in its formation. It may extend at first over five or six 

 somites, bnt the part beyond the anterior three soon aborts. In some 

 cases it has a greater extension on one side of the body than on the 

 other. 



Three funnels seem, as a rule, to be formed on each side of the 

 body, but tbe most posterior of these disappear, two pairs being left. 

 These persist throughout the larval period. The ciliated openings 

 into the body-cavity become narrowed. By this narrowing of the 

 opening, and the widening of the part opposite to the glomerulus, the 

 pronephric chamber described by Balfour and Parker arises. It need 

 scarcely be remarked that there are two of these "chambers" on 

 each side. Even when the mesonephros is in course of development, 

 i.e., from the 16th to the 18th day, the two funnels on each side still 

 persist and are quite distinct. 



As just stated, the mesonephros arises between the 16th and 18th 

 day. The date of its development seems to be variable. Its tubes 

 are formed in the angle of the body-cavity between the region of 

 the segmental duct and the genital ridge ; the latter is at this period 

 part of the mesentery of the alimentary canal. One notices that the 

 cells of this region of the ccelom are filled with yolk. These yolk- 

 filled cells give rise to the mesonephros and the genital glands. The 

 storing-up of yolk reminds one of Dohrn's discovery of yolk-laden 

 cells in the head- somites of Ammoccetes, i.e., in those structures 

 which at a much later period form the eye-muscles of the Petromyzon. 

 At the 16th day the somites have been almost entirely converted 

 into muscle and connective tissue, and in Lepidosteus the mesonephros 

 can neither be derived from the part connecting the somites and 

 body-cavity together, as in Elasmobranchs and birds (Sedgwick; van 

 Wijhe), nor from part of the somites (nephrotome of Riickert). 

 There is here no intermediate cell-mass as in the chick, but the 

 mesonephric tubules arise as distinct segmental evaginations of the 

 wall of the body-cavity at the point indicated above. They grow 

 over the segmental duct in a curved fashion, and open into it by 

 piercing through its wall. 



Such is the brief account of the early development of Lepidosteus 

 which I feel at present in a position to give, and I close this communi- 

 cation with a notice of — 



A Transient or Larval Nervous Apparatus in Lepidosteus and certain 

 other Ichthyopsida. 



In the course of my investigations I noticed the very frequent 

 occurrence of " giant ganglion cells " in a particular situation along 



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