Development of the Fourth Day 241 



it forms into rod-shaped masses which are 

 separated by septa of mesoblast derived from 

 the patch of thickened mesoblast underneath. 

 These rods of epithelial cells probably become 

 converted into the seminiferous tubules, 

 though the exact way in which this occurs is 

 not clearly understood. The development of 

 the spermatozoa (spermatogenesis) from the 

 cells of the seminiferous tubules will be found 

 described in larger books of embryology, or 

 in text-books of histology. 



The heart. — The heart undergoes some 

 marked changes during this day (Fig. 75). 

 The pointed loop which will form the apex of 

 the ventricles still projects somewhat towards 

 the right, but it is coming to point in a more 

 ventral direction. Well-marked constrictions 

 now separate the ventricles from the auricles, 

 on one side, and from the bulbus, on the other 

 (Fig. 75). Although there is no external in- 

 dication of its formation, there is developed, 

 during this day, an incomplete septum divid- 

 ing the ventricle into two chambers. The 

 septum being, as yet, incomplete the two 

 chambers of the ventricle communicate freely 

 with each other. The bulbus arteriosus and 

 the auricles have increased in size, and the 



