82 THE SECOND DAY. [CHAP. 



21. At the latter end of this day, the ridge which we 

 have already spoken of as the rudiment of the Wolffian duct, 

 has become distinctly hollow, is in fact no longer a ridge but 

 a canal. Sections now shew not an irregular group of ordi- 

 nary mesoblastic cells, but a small cavity surrounded by a 

 wall of cells ; and these cells are beginning to put on a 

 columnar character, and thus appear to radiate from the cen- 

 tral cavity. The canal or duct so formed, the anterior 

 termination of which is closed, and the posterior not as 

 yet completely formed, reaches from about the fifth pair 

 of proto vertebrae backwards towards the hind end of the 

 embryo. The conversion of the ridge into a canal appears 

 to take place by the cells acquiring a radiating arrange- 

 ment, and a small hole appearing at the centre where the 

 points of the cells meet ; this rapidly grows larger till it 

 reaches the final size of the cavity of the duct. 



The exact mode of development of the Wolffian duct is still a matter of some 

 doubt, although its origm has been investigated by numerous embryologists. 



Eemak, and after him KoUiker, described it as taking its origin from the 

 mesoblast of the soraatopleure, and appearing about the middle of the second 

 day, at the external border of the protovertebrae immediately under the epi- 

 blast, in the form of a solid cord which subsequently bei-ame hollow. 



Dursy (Ztitsch. f. Rat. Med. 1S65) gave a very similar account, except that 

 he regarded it as being derived from the substance of the protovertebrae, 

 instead from the somatopleure. 



Hensen (4rc/«'!', Microscop. Anat. Ed. ill. 1867), and for some time His, 

 believed tliat the duct took origin as a longitudinal involution of the epiblast 

 between the protovertebrae and the lateral mesublast, in the form of a groove, 

 which subsequently became closed in and detached from the superficial epiblast, 

 in a manner very similar to the way in which the lens is formed. 



Subsequently His took up the view that it was a product of the proto- 

 vertebrcB, the central cells of these bodies, according to him, protruding as a 

 ridge along their upper and external angles. He states that at first a distinct 

 connection ia visible between the Wolffian duct and the central cells of the 

 protovertebrse. 



Waldeyer (Eierstoch u. Ei, 1870) has given a totally different account. 

 Between the external border of the protovertebrae, and the point where the 

 mesoblast splits into somatopleure and splanchnopleure, there lies a mass of 

 cells, which we shall have occasion to speak of hereafter as the intermediate cell- 

 mass. According to Waldeyer, the upper surface of this mass grows up into 

 a narrow ridge, seen in sections as a tongue-shaped process projecting into the 

 vacant space (i. e. the space fillfcd with fluid only) which exists below the 

 epiblast at this point. Later on, this tongue-shaped process is seen to curve out- 

 wards, and thus to become hook-shaped ; and the point of the hook subsequent y 

 unites with a similar smaller process derived from the more external portions 

 of the same cell-mass. The small cavity thus seen to be enclosed by a larger 

 and smaller process, is of course the sectional view of a canal enclosed by a 

 larger and smaller ridge. This canal is the Wolffian duct. Waldeyer further 

 believes that the cells which thus form the walls of the duct are primarily epi-. 



