Outlines of zoology . ion. The most important characteristic of the reproductive cells, whether male orfemale, is that theyretain the essentialqualities of the fer-tilised ovum fromwhich the parentanimal was devel-oped. The ovum has theusual characters of acell; its substance istraversed by a fineprotoplasmic net- work; its nucleus or Fig. , showing diffuse ge™inal vesicle con-yolk granules. tams the USUal chro- , Germinal vesicle or nucleus ; chr., chromatin ™^tin elements J It elements. has often a Store of reserve material oryolk, and a distinct sheath representing a ce
Outlines of zoology . ion. The most important characteristic of the reproductive cells, whether male orfemale, is that theyretain the essentialqualities of the fer-tilised ovum fromwhich the parentanimal was devel-oped. The ovum has theusual characters of acell; its substance istraversed by a fineprotoplasmic net- work; its nucleus or Fig. , showing diffuse ge™inal vesicle con-yolk granules. tams the USUal chro- , Germinal vesicle or nucleus ; chr., chromatin ™^tin elements J It elements. has often a Store of reserve material oryolk, and a distinct sheath representing a cell wall(Fig- 27)- In Sponges the ova are well-nourished cells in the middlestratum of the body; in Coelentera they seem to arise inconnection with either outer or inner layer (ectoderm orendoderm); in all other animals they arise,in connectionwith the middle layer or mesoderm, usually dn an area ofthe epithelium lining the body cavity^- In Ipwer anima;lsthey often arise somewhat diffusely; it^ higher animals their. EMBRYOLOGY. 59 formation is restricted to distinct regions, arid usually todefinite organs—the ovaries. The young ovum is often amoeboid, and, that of Hydraretains this character for some time (Fig. 70, p. 148). Theovum grows at the expense of adjacent cells, or by absorb-ing material which is contributed by special yolk glands orsupplied by the vascular fluid of the body. The yolk or nutritive capital may be small in amount,and distributed uniformly in the cell, as in the ova ofMammals, earthworm, starfish, and sponge; or it may bemore abundant, sinking towards one pole as in the egg ofthe frog, or accumulated in the centre as in the eggs ofInsects and Crustaceans; or it may be very copious, dwarf-ing the formative protoplasm, as in the eggs of Birds,Reptiles, and most Fishes (Fig. 31). Round the egg there are often sheaths or envelopes ofvarious kinds—{a) made by the ovum itself, and then verydelicate { the vitelline membrane); {b) formed by ad-jacent c
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
