256 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



The later development of the individual nephrotome of the nieso- 

 nephros takes place in the Birds along lines exactly similar to what 

 takes place in lower forms such as the Amphibia. The tubule rudiment 

 originates as an, at first solid but later pocket-like, outgrowth of the 

 lateral wall of the nephrotome (Fig. 137, C, t). The tip of this 

 presses against the mesial wall of the duet and, as the tubule grows in 

 length, fusion takes place and the lumina of duct, tubule rudiment 

 and nephrotome — which together form a characteristic ru-shaped 

 structure as seen in a transverse section — become continuous (Fig. 

 137, D and E). The portion of the oj nearest the mesial plane 

 represents the nephrotome in the strict sense, i.e. the forerunner of 

 the Malpighian body, and has assumed a watch-glass shape, its 

 dorsal wall being involuted into the cavity as the rudiment of the 

 glomerulus {glom). 



The further development of the mesonephric unit, which need 

 not be followed out in detail in this book, consists in (1) the immense 

 growth in length of the tubule, which leads to its becoming in- 

 extricably intertwined with its neighbours, (2) the histological 

 differentiation of its wall, and (3) the differentiation of the Malpighian 

 body. 



It should be mentioned that where the tubules are much crowded 

 together they do not all establish a communication with the duct 

 in the typical manner above described. Some, even of the primary 

 tubules, come to open into neighbouring tubules. In the case of the 

 subsequent tubules, some open into the duct in the typical fashion, 

 others open into neighbouring tubules, while the majority become con- 

 nected with pocket-like outgrowths from the duct. These outgrowths 

 are greatly developed in some birds (cf. Duck, Fig. 138, B), becoming 

 much elongated and taking the form of branched collecting-tubes into 

 each of which open a series of subsequent tubules (cf. Hypogeophis), the 

 whole condition distinctly foreshadowing arrangements presently to be 

 mentioned in the metanephros. 



The mesonephros acts as the renal organ only for a short period 

 during the early stages of development. In the Fowl it begins to 

 develop about the end of the second day of incubation, it reaches 

 its maximum about the 7th or 8th day, and almost immediately 

 thereafter begins to show signs of degeneration as the renal func- 

 tion becomes concentrated in the metanephros. The mesonephros 

 never completely disappears though it ceases to be of any importance 

 as a renal organ : its persistence is correlated with the fact that 

 this portion of the opisthonephros has already in the forerunners 

 of the Amniota important functions connected with reproduction. 

 Its modification in relation to these functions will be gone into 

 later. 



Metanephros of Birds. — The continuous mass of mesen- 

 chymatous tissue representing the nephrotomes or protovertebral 

 stalks does not cease at the hinder limit of the mesonephros at 

 segment XXX : it is continued on through segments XXXI, XXXII, 



