58 REPRODUCTION AND LIFE HISTORY. 



in the history of some " worms " and Tunicates. They 

 illustrate a rhythm between asexual and sexual multiplica- 

 tion, between parthenogenetic and normal sexual reproduc- 

 tion, between vegetative and animal life, between a relatively 

 " anabolic " and a relatively " katabolic " preponderance. 



IT. EMBRYOLOGY 



Egg cell or ovum. Apart from cases of asexual repro- 

 duction and parthenogenesis, every multicellular animal 

 begins life as an egg cell with which a male cell or sperma- 

 tozoon has entered into intimate union. 



The most important characteristic of the reproductive 



cells, whether male or 

 female, is that they 

 retain the essential 

 qualities of the fer- 

 tilised ovum from 

 which the parent 

 animal was devel- 

 oped. 



The ovum has the 

 usual characters of a 

 cell ; its substance is 

 traversed by a fine 

 protoplasmic net- 



_ work; its nucleus or 



FIG. 27. -Diagram of ovum, showing diffuse g e inal vesicle COn- 

 yolk granules. tains the usual chro- 



g-.v., Germinal vesicle or nucleus ; chr. t chromatin matm elements ; it 



elements. has often a store of 



reserve material or 



yolk, and a distinct sheath representing a cell wall 

 (Fig. 27). 



In Sponges the ova are well-nourished cells in the middle 

 stratum of the body ; in Ccelentera they seem to arise in 

 connection with either outer or inner layer (ectoderm or 

 endoderm); in all other animals they arise in connection 

 with the middle layer or mesoderm, usually on an area of 

 the epithelium lining the body cavity. In lower animals 

 they often arise somewhat diffusely ; in higher animals their 



