156 CHILD. 



very strong evidence in favor of the view that the principal parts 

 of the system develop independently. A glance over the figures 

 will show that almost any portion of the organs may be present 

 alone. As noted above, the middle portions of the ducts never 

 appear alone and it may be that the ovary and vitellarium always 

 appear together. The dependence of the middle regions of the 

 ducts on other portions has been discussed. The relation of 

 ovaiy and vitellarium is perhaps explicable on the view com- 

 monly held that the vitellarium represents a modified portion of 

 the ovaiy. The earliest stage of the reproductive organs is a 

 mass of nuclei in the region where the ovary and vas deferens 

 appear later. This increases in size and gradually differentiates, 

 but the study of the abnormalities renders it very evident that the 

 development of the portion which forms the vas deferens does 

 not depend upon the development of the ovarian portion and 

 vice versa, for the one portion ma}- appear without the other, 

 although both appear to differentiate from a common mass. 

 Testes and vas deferens develop independently of each other, for 

 testes appear in proglottids where there are no vasa deferentia 

 (Fig. 39, b ; Pt. II.) and vice versa. And finally, the pore and 

 terminal portions of the ducts develop independently of the other 

 organs. In the normal proglottids the group of nuclei represent- 

 ing the pore, cirrus, etc., is entirely unconnected in early stages 

 with the group representing the inner organs and may appear 

 alone (Fig. 19, b ; Pt. I.). The connecting portions of the ducts 

 appear later, as was mentioned above. 



We must conclude, therefore, that the lateral reproductive or- 

 gans are formed from at least three distinct " centers " or regions 

 of independent growth. These may be called the vas deferens 

 region, the ovarian region, and the terminal region. There may 

 be others besides these. In addition, the testes develop in the 

 central parenchyma independently of each other and of the lateral 

 organs. All cases of the incomplete development of the repro- 

 ductive organs are to be regarded as due to the incomplete de- 

 velopment or total obliteration of one or more of these regions. 



It is important to note that in all cases of partial development 

 the portions of the reproductive system present show approxi- 

 mately the same degree of differentiation as the corresponding 



