IV.] THE WOLFFIAN DUCT. 73 



the embryo ; but all are considerably smaller and also somewhat more 

 columnar than the more peripheral cells of the pellucid area. The largest 

 epiblastic cells are to be found in the region of the vasnular area, but here 

 they are much flattened. At the extreme outer edge of the opaque area the 

 cells are smaller again, shewing in this respect a marked contrast to their con- 

 dition during the previous stage. 



8. About this period there may be seen in transverse 

 sections, taken through the embryo in the region of the proto- 

 vertebrse, a small group of cells (Fig. 20, W. d) projecting on 

 either side from the mass of uncleft mesoblast on the outside 

 of the protovertebroe, into the somewhat triangular space 

 formed by the epiblast above, the upper and outer angle of the 

 protovertebra on the inside, and the mesoblast on the outside. 



This group of cells is the section of a longitudinal ridge, 

 the rudiment of the Wolffian duct. We shall return to it 

 immediately. 



9. The most important changes then which take place 

 during the first half of the second day, are the closure of the 

 medullary folds, especially in tbe anterior part, and the 

 dilatation of the canal so formed into the first cerebral 

 vesicle ; the establishment of a certain number of protoverte- 

 brae ; the elevation of the head from the plane of the 

 blastoderm ; the formation of the tubular heart and of the 

 great blood-vessels; and the appearance of the rudiment of 

 the Wolffian duct. It is important to remember that the 

 embryo of which we are now speaking is simply a part of the 

 whole germinal membrane, which is gradually spreading over 

 the surface of the yolk. It is important also to bear in mind 

 that all that part of the embryo which is in front of the most 

 anterior protovertebrse corresponds to the future head, and 

 the rest to the neck, body and tail. At this period the head 

 occupies nearly a third of the whole length of the embryo. 



10. The changes which take place from the 36th to 



the 45th hour will best form the next stage, since those 

 which occur during the last few hours of the second day will 

 be more conveniently described with the third day. 



One important feature of the stage is the rapid increase 

 in the process of the folding off' of the embryo from the plane 

 of the germ, and its consequent conversion into a distinct 

 tubular cavity. At the beginning of the day, the head alone 

 projected from the rest of the germ, the remainder of the 



