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. 
II. 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 
germinal vesicle con- 
Fic. 27.—Diagram of ovum, showing diffuse 
yolk granules. tains the usual chro- 
g.v., Germinal vesicle or nucleus ; chv., chromatin matin elements 3 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. 
