THE 



ANATOMY OF VERTEBRATES. 



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CHAPTER I. 



CHARACTERS OF VERTEBRATES. 



§ 1. Developmental characters. — Vertebrates, like lower animals, 

 begin in a semifluid nitrogenous substance called ' plasma,' fig. 1 , A, 

 a ; primarily differentiating into albumen, fibrine, lemma, ib. 0, c x } 

 nuclei and cells : in which lat- l 



ter form the individuality of 

 the new organism first dawns 

 as a nucleated ' germ-cell ' or 

 germinal vesicle, ib. d. 



By the evolution of albumi- 

 nous granules and oil-particles 

 plasma becomes ' yolk,' fig. 1, 

 b, c ; the germinal vesicle may 

 be obscured by endogenous 

 multiplication of granules,gra- 

 nular cells and oil-globules, 

 which combine with those of 

 the yolk to form its germinal 

 part : an outer layer of i lem- 

 ma,' D, ch, completes the un- 

 impregnated vertebrate egg. 



For further developement 

 another principle is needed, 

 viz. the hyaline nucleus or 

 product of the sperm-cell, fig. 



2, called ' spermatozoon.' Its reception by the egg, as at a, b, fig. 



3, is followed by the formation of a germ-mass. This mass is due 



1 Gr. lemma, skin ; also called 'primary' or 'basement' membrane ; distinguished, 

 through its relations, as 'neurilemma, sarcolcmma, adenolemma' or the limitary 

 membrane of gland-follicles, &c. 



Stages of developement of the ovarian egg of a vertebrate 

 animal {Gasterosteus). clxxvi. 



