310 ESSENTIALS OF ZOOLOGY 



preserved and frequently accentuated which further indicate 

 this. In the male, between the vasa deferentia at their terminal 

 ends in the urethra a uterus masculinus is practically always 

 present in mammals as a remnant of the Miillerian duct, and 

 sometimes the inner end of the Miillerian duct may be traced 

 in close proximity to the epidid3^mis. In the female, similarly 

 the broad ligament presents in the neighbourhood of the ovary 

 remnants of the Wolffian duct and of the vasa efferentia. This 

 goes to show that at the early period the endocrines which 

 provide the stimuli for the growth of the organs are at first 

 indifferent, and later become specific with respect to sex. 



Germ Cells. The essential elements of the reproductive 

 system are the germ cells. They appear sooner or later in 

 development : sooner in animals which quickly resolve their 

 relatively few cells by differentiation, and later in animals 

 which are able to preserve for a longer period cells in an indif- 

 ferent state. But whether they appear early or late, whether 

 they first become visible to us in ectoderm, endoderm, or meso- 

 derm, is of little importance, for they are from their first appear- 

 ance separate entities which maintain a single-celled state in 

 the soma. It has been convenient in dealing with the verte- 

 brates to speak of them arising in the germinal ridge. The 

 fact is, however, that they may be recognised outside the ridge 

 and that they migrate into the ridge. Independent as they 

 are, we thus see that they are attracted in the soma to a special 

 region, and we have no hesitation in ascribing the attraction to 

 some chemical substance secreted by the somatic cells of the 

 region. The result is the formation of an ovary or a testis, or 

 both. 



In the ovary and in the testis the germ cells multiply, 

 maintaining, however, their individuality and conducting them- 

 selves as Protozoa in a state of protection. No difference is 

 manifested, as a rule, during this period of multiplication 

 between the testis and the ovary. The result is the provision of 

 a number of germ cells, a number which appears to be specific. 

 No further change takes place except one of growth until the 

 period of maturity, when two successive divisions take place, 

 resulting respectively in the formation of ova and spermatozoa. 



In the case of many aquatic animals, up to and including 



