The Birth-period of Trichosurus vulpecula. 33 



and resembles very strikingly that of a Scyllium-emhryo slightly over 

 the critical phase. 



In the spinal cord the formation of the posterior fissure, or 

 septum, has only proceeded a slight distance, about as far as in a 

 Scyllium canicula embryo of 34 mm, or as in a rabbit-, or mole- 

 embiyo, just over the critical phase. 



The permanent kidney and ureters are fairly well-developed, and 

 tubuli contorti are present. The Wolffian body or raesonephros is 

 still persistent — according to Selenka its activities continue in the 

 opossum for some weeks after birth. 



The sex is established, and the specimen under description is a 

 male. The canal of the penis is not yet perforated, it is filled with 

 cells. The Miillerian ducts are formed for part of their course. The 

 rectum is open. 



The foundations of a mammary apparatus are present (Fig. 5). 

 The epipubic cartilages appear to be present. The hind hmbs and 

 hinder portion of the body show hardly an appreciable advance on the 

 younger embryo. The cells of the umbilical cord are not shrivelled up. 



The characters of the two embryos have been described in much 

 greater detail, than is usual in embryological research; but, undoubtedly, 

 there are some points, about which nothing has been said. In explanation 

 it may be stated, that the preparation of the sections entailed so much 

 labour, that, after they were ready for examination, only a few evenings 

 could be devoted to their study, when they left my possession. Few 

 embryologists have ever been in a position to work through such 

 series of marsupials at all, and, therefore, one ought to be sufficiently 

 grateful for the opportunity, all too short in its duration, yet long 

 enough to establish the essential facts. And, indeed, this latter was 

 all, that I wished to be able to do. No doubt, had more time been 

 available, greater detail might have been aimed at, though in essentials 

 the results would have remained the same. 



In many respects the present short paper will be found to sup- 

 plement and complete Selenka's fine description of late uterine and 

 newly-born opossums, and it can claim to be an advance on his work, 

 in that, for the first time, it becomes possible to compare together in 

 all essential points a marsupial embryo just prior to birth and a 

 newly-born form. 



In order to render this comparison easier, and to make additions 



6* 



