THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 439 



and testicles, true hermaphroditism is the result; if they give rise to either 

 ovaries or testicles but the external genitalia and general characteristics develop 

 in the opposite direction, false hermaphroditism is the result. Thus the her- 

 maphroditic condition is potentially present in every individual during the 

 earlier stages of development; the most remarkable fact is that it is not more 

 common. 



Recent researches in cytology have added a new phase to the question of the 

 origin of hermaphroditism. Accessory chromosomes have been demonstrated 

 in the ova and spermatozoa of many species of insects (McClung, Wilson, 

 Morgan) and in ova and pollen of dioecious plants (Correns). It has been 

 suggested that these have some significance in the determination of sex, the 

 female elements containing the additional chromatin elements (see p. 416). 

 Carrying this a step further, Adami has suggested that "hermaphroditism is 

 based upon aberration in the distribution of the chromosomes in either the ovum 

 or the spermatozoon." 



PRACTICAL SUGGESTIONS. 



The Pronephros. Being a very rudimentary structure in the higher Vertebrates (especially 

 in Mammals), the pronephros can be studied to the best advantage in embryos of some of 

 the lower forms, such as the frog. The embryos can readily be obtained in the spring in 

 ponds; or perhaps a better method is to collect the eggs during the early cleavages (which can 

 be seen with the naked eye or with the aid of a hand lens) and allow them to develop in 

 water in the laboratory, selecting successive stages for preservation. Beginning when the 

 embryos are three to four mm. long, remove the gelatinuous capsules and fix in Flemming's 

 fluid. Successive stages up to any length desired (twelve to fifteen mm.) should be prepared 

 in the same manner. Cut transverse serial sections in paraffin and stain with Heidenhain's 

 haematoxylin. The embryos of four to five mm. will show the early stages, longer ones, of 

 course, the later stages, those of about 12 mm. being especially good. The developing 

 pronephroi are found in the mesoderm lateral to the primitive segments. The ducts, in 

 the same relative position, can be traced through the series of sections to the tail region 

 where they then bend ventrally and medially to open into the gut. For technic, see Appendix. 



The Mesonephros. Chick embryos at the end of the second and during the third and 

 fourth days of incubation are most convenient for the study of the early development of the 

 mesonephros. Remove the embryo from the egg, fix in Zenker's fluid, cut transverse serial 

 sections in paraffin and stain with Weigert's haematoxylin and eosin. Time can be saved by 

 staining in Mo with borax-carmin, but the differentiation is less complete. The mesonephroi 

 appear in the intermediate cell mass at about the level of the heart, development thence 

 progressing caudally. The mesonephric ducts, which are identical with the pronephric 

 ducts, can be traced caudally until they turn ventrally and medially and open into the caudal 

 end of the gut. 



In these embryos the formation of the glomeruli can be studied, likewise the increasing 

 intimacy between the posterior cardinal and subcardinal veins and the mesonephric tubules. 

 By examining series of sections, the segmental mesonephric branches of the aorta can be 

 seen. 



The mesonephroi at the height of their development can be studied in human embryos 



