160 THE FOURTH DAY. [CHAP. 



in the lower vertebrates, such as fishes, where we find that the lateral muscle is 

 divided by septa into a series of segments corresponding in number with the 

 vertebrae. 



17. Of all the events of the fourth day, none perhaps are 

 more important than those by which the rudiments of the 

 complex urinary and generative systems are added to the 

 simple Wolffian duct and body, which up to that time are 

 the sole representatives of both systems. 



We saw that the duct arose on the second day as a solid 

 ridge which subsequently became a tube, lying immediately 

 underneath the epiblast above the intermediate cell-mass, 

 close against the upper and outer angles of the proto- 

 vertebrse, and reaching from about opposite to the fifth 

 protovertebra away to the hinder end of the embryo. 



The exact manner in which it first appears is as yet a 

 matter of dispute, and in our account of the second day, we 

 gave the views of the majority of embryologists who have 

 written on the subject. But it may be considered as quite 

 certain that the Wolffian duct is formed out of mesoblast- 

 cells. It is most probable that the ridge is primarily formed 

 by simple aggregation of cells, and that it is converted into 

 a tube by its central cells taking on a radiating arrange- 

 ment round a central hole, which is at first small but rapidly 

 increases in size. In whatever way it be really formed, we 

 find before the end of the second day, in the place of the 

 previous ridge, a duct with a distinct though small lumen. 

 Waldeyer and some other observers have incorrectly stated 

 that the lumen is not formed till somewhat later. 



At first the duct occupies a position immediately under- 

 neath the superficial epiblast, but very soon after its forma- 

 tion the growth of the protovertebrae and the changes which 

 take place in the intermediate cell-mass, together with the 

 general folding in of the body, cause it to appear to change 

 its place and travel downwards (Chap. V. 26). While this 

 shifting is going on, the cells lining the upper end of the 

 pleuroperitoneal cavity (the kind of bay which, as seen in 

 sections, is formed by the divergence of the somatopleure and 

 splanchnopleure) become columnar, and constitute a distinct 

 epithelium. This epithelium, which is clearly shewn in Fig. 

 41, g. e, and is also indicated in Fig. 44, is often called i\\Q ger- 

 minal epithelium, because some of its cells subsequently take 



