402 PHILOSOPHY OF ZOOLOGY. 



had no doubt of the other uterus containing a similar em- 

 bryo in a less advanced state." 



The origin of this gelatinous substance has not been sa- 

 tisfactorily ascertained. As the coats of the uterus are thin, 

 it is scarcely to be considered as a secretion from these, but 

 is more likely to proceed either from the oviducts or late- 

 ral canals. But the manner in which the foetus is nourished 

 in this jelly, is a question which remains to be determined. 



The evolution of the ovum by means of placentation, is 

 the most common method observed by Nature, and has 

 been longest known and studied by physiologists. The 

 ovum is first attached to the walls of the uterus, which are 

 lined for this purpose with a layer of lymph, termed mem- 

 brana decidua ; a vascular body termed a placenta, is then 

 generated, in which the minute branches of the enlarged ute- 

 rine arteries terminate, and from which the vessels destined 

 to convey the blood for the nourishment of the foetus take 

 their rise. The blood is returned from the foetus by other 

 vessels which terminate in the placenta. These vessels, 

 which unite the foetus with the placenta, are collected into 

 the umbilical cord or Navel-string, so named in consequence 

 of the form exhibited, and the place of insertion in the foe- 

 tus. At the commencement of this evolution, the os tincse 

 is obstructed by a glairy matter, which, while it closes the 

 aperture, prevents the adhesion of the margins. But, in 

 order to illustrate this curious subject still farther, it will 

 be necessary to return to the structure of the ovum. 



The ovum, after it has sufficiently increased in size, ap- 

 pears to be surrounded by two membranes or involucra. 

 The innermost one is termed amnios, and the external one 

 chorion. 



The amnios is destined to secrete a peculiar fluid, term- 

 ed liquor amnii, with which the foetus is surrounded. This 



