HISTORY OF THE HUMAN BODY 



the micro-gametes, first discovered in the seminal fluid of 

 mammals,* were termed spermato-zoa, or sperm animals, a 

 term expressing the view held at that time that they were 

 parasitic or adventitious organisms occurring in a fertilizing 

 or quickening fluid, and from this the act of mixing the 

 spermatic fluid with the ova was termed fertilization. This term 

 is now applied technically to the entrance of the spermatozoon 



FIG. 9. Earliest stages of Metazoan development. 



The upper row represents the egg of Sycandra, a calcareous sponge [after F. E. 

 SCHULZE] ; the lower row represents that of the rabbit [after BISCHOFF]. In the 

 rabbit the egg is surrounded by a thick capsule, the zona pellucida. The egg of the 

 sponge is without this and floats freely in the water. 



into the ovum, i. e., to the union of the two gametes, and is 

 thus synonymous with conjugation,, when applied to Metazoa. 

 Furthermore, since, in the majority of cases, the bulk of the 

 ovum so far exceeds that of the spermatozoon that the latter 

 appears to be lost in the process, the term ovum, or egg, is com- 

 monly used to designate not only the macro-gamete (the un- 

 fertilized egg), but also the double cell resulting from the 

 conjugation (the fertilized egg), a use of terms which neces- 

 sitates constant watchfulness in order to guard against con- 

 fusion. Ovum and spermatozoon, the macro- and micro-ga- 



* Discovered in 1677 by Ludwig Hamm, a pupil of Leeuwenhoek. 



