246 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



development from about the eighth day of incubation ; it never 

 opens into the cloaca, and it persists in the adult as a functionless 

 vestige. 



There is considerable probability that the genital pores paired 

 openings leading from the hinder end of the splanchnocoele directly 

 into the urinogenital sinus (Cyclostomata) and through which the 

 gametes pass out are to be looked on as Miillerian ducts in the 

 last stage of reduction, the whole duct having disappeared except 

 its hinder opening. Whether there is any evidence bearing on this 

 in their ontogeny is not yet known. 



The Miillerian duct goes through the early stages of development 

 in the male as well as in the female. It usually however never opens 

 into the cloaca and it soon becomes reduced to a vestige. This may 

 persist to a greater or less extent as an individual variation or as a 

 normal characteristic, e.g. in the male Elasmobranch or Lung-fish 

 well-marked vestiges remain in the adult, and so, still more markedly, 

 in some of the Amphibia such as the Bufonidae and some of the 

 Gymnophiona. 



OPISTHONEPHROS. Here again Brauer's excellent account of the 

 development in Hypogeophis (1902) may be taken as a basis of our 

 description. The opisthonephros in this amphibian is composed of 

 segmentally arranged units extending from segment 24 to segment 

 100. Each unit is identical in composition with those of the 

 pronephros, consisting of a tubule and a chamber (Malpighian body) 

 containing a glomerulus and communicating with the splanchnocoele 

 by a peritoneal canal. As in the case of the pronephros, each unit 

 arises in development from the nephrotome or protovertebral stalk, 

 the tubule rudiment being in the form of a diverticulum of the 

 lateral or somatic wall of the nephrotome, the blind end of which 

 comes in contact and fuses with the wall of the duct secondarily. 

 Again as in the case of the pronephros, the nephrotome becomes 

 completely separated from the myotome. It also becomes constricted 

 off from the splanchnocoelic mesoderm, incompletely in some cases, 

 a narrow communication the peritoneal canal remaining open 

 between the nephrocoele and the splanchnocoele, but more usually 

 completely. In this latter event a new peritoneal canal is developed 

 secondarily in place of that which has been obliterated, a diverticulum 

 growing out from the wall of the iiephrotomr \\hirh meets and fuses 

 with the Bplanchnoooelic epithelium. 



There are differences in detail between the de\vlo]>ment of 



pronrphros and opislhonephros, e.g. the tubule rudiment makes its 



appearance relatively later in the case of the latter at a period after 



the nephrotoim- has become constricted off from the splaiirlmoeorlie 



id-Hi. A furthrr diffrrenr.- lirs in the fart llial there takes 



in ihr OpiflthonephrOB a -real inereasr in the number of its 



tubules secondary, tertiary, eta tubules brill-- added to thosr of the 



ari e in characteristic fashion, An out-To\\ih 



hum t.li.- :urdiaii portion of the nrphrotome and 



