STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 823 



cavity has been studied by Rosenberg 1 and Gotte 2 . According 

 to the account of the latter, which we have not ourselves con- 

 firmed but which has usually been accepted, the front end of the 

 segmental duct, instead of becoming folded off from the body- 

 cavity, becomes included in a kind of diverticulum of the body- 

 cavity, which only communicates with the remainder of the 

 body-cavity by a narrow opening. On the inner wall of this 

 diverticulum a projection is formed which becomes a glomerulus. 

 At this stage in the development of the pronephros we have 

 essentially the same parts as in the fully formed pronephros of 

 Lcpidosteus, the only difference being that the passage con- 

 necting the diverticulum containing the glomerulus with the 

 remainder of the body-cavity is short in Teleostei, and in Lepi- 

 dostens forms a longish ciliated canal. In Teleostei the opening 

 into the body-cavity becomes soon closed. If the above com- 

 parison is justified, and if the development of these parts in 

 Lcpidosteus takes place as it is described as doing in Tele- 

 ostei, there can, we think, be no doubt that the ciliated canal 

 of Lcpidosteus, which connects the pronephric cavity with 

 the body-cavity, is a persisting communication between this 

 cavity and the body-cavity; and that Lepidosteus presents 

 in this respect a more primitive type of pronephros than 

 Teleostei. 



It may be noted that in Lcpidosteus the whole pronephros 

 has exactly the character of a single segmental tube of the 

 mesonephros. The pronephric cavity with its glomerulus is 

 identical in structure with a malpighian body. The ciliated 

 canal is similar in its relations to the peritoneal canal of such a 

 segmental tube, and the coiled portion of the pronephros re- 

 sembles the secreting part of the ordinary segmental tube. This 

 comparison is no doubt an indication that the pronephros is 

 physiologically very similar to the mesonephros, and so far 

 justifies Sedgwick's 3 comparison between the two, but it does 

 not appear to us to justify the morphological conclusions at 



^Rosenberg, Untersitch. neb. d. Enhvick. d. Tdeosticrnicre, Dorpat, 1867. 



2 Gotte, Entwick. d. Unite, p. 826. 



3 Sedgwick, " Early Development of the Wolffian Duct and anterior Wolffian 

 Tubules in the Chick; with some Remarks on the Vertebrate Excretory System," 

 Quart. Journ. of Micros. Science, Vol. xxi., 1881. 



