338 DR D. BERRY HART. 



in all probability in a way miich used in the developing embryo, 

 as we have seen in the development of the anus, and shall see 

 when we take up the development of the prepuce and vagina. 

 The active ectoderm by prohferation forms a plug, cylindrical 

 or pyramidal, the deeper active cells forming the outer surface 

 except at the free base, the centre of the plug or cylinder 

 being made up of cells less active and tending to be shed. 

 Thus, after a plug has formed, the centre breaks down, and a 

 €avity or slit forms. This is cavity — or slit — formation by ecto- 

 dermic action, and is sharply contrasted with the method of 

 formation of cavities by splanchnopleuric folds where no break- 

 ing down happens. As a broad fact, therefore, the ectoderm 

 forms orifices or open slits, by the breaking down of a solid 

 plug ; the mesoderm, cavities or lumina by partitions (figs. 6-9 

 and 10, 11, 18 and 19). 



III. The Changes at the 14th Week and at the 24th. 



The changes that take place at or about the 14th week are 

 of very great importance and of the highest interest. We 

 undoubtedly need graduated specimens filling up the stages 

 between the 8th and 14th week, but even with these gaps not 

 yet made good, the changes at the 14th week are of a very 

 evident and pronounced nature. Thus, on viewing a mesial 

 section of a female foetus at the 14th week, and comparing it 

 with that at the 7th week, we see that the cloacal condition has 

 disappeared, the urinogenital sinus has shortened, the hymen 

 is in process of formation, the relations of urethra and vagina 

 are more like those of the adult condition, a permanent anus is 

 in course of formation, and the prepuce of the clitoris is being 

 developed. 



The mechanism of the formation of the rectum and bladder, 

 of their external openings and the formation of the permanent 

 anus, have already been given (pp. 334-337), and need not be 

 discussed. We must take up, however : 



1. The formation of the preputium clitoridis. 



2. The development of the hymen, and the changes in the vagina 



and uterus. 



