IN THE ANIMAL KINGDOM. 73 



cess of cell-formation occurs in tlie ova of Pyrosoma and 

 Salpa.* 



The embryo — whether the direct or indirect product of 

 impregnation — differs in the majority of cases from tlie 

 adult form, not only in size, but in many points also of 

 structure and configuration, and the progressive changes 

 which it undergoes before acquiring perfect conformity to 

 the organization of the parent go under the name of meta- 

 morphosis. Sometimes they occur within the embryonic 

 envelopes, but at other times the young is extruded while 

 still in an imperfectly developed state. Such naked em- 

 bryos are termed larva? — masks, as it were, disguising 

 what is ultimately found to be the true aspect of the species. 



In some Invertebrata, Fishes, and Reptiles, and still more 

 strikingly in Birds, the segmentation appears not to extend 

 to the whole contents of the ovum, but the exception is 

 more apparent than real, for the yolk of the fertilized egg- 

 in such cases contains part of the granular matter within 

 the ovisac, over and above the proper substance of the 



* Annals of Nat. History, 3d Ser. (Jany., 1860), p. 35. There is per- 

 haps more force in Mr. Huxley's other suggestion of an analog}' with the 

 Bird's ovum, though in this view the part wanting would not he the true 

 cleaving yolk, hut the wall of the proper germinal vesicle, and the true 

 ovum would he represented hy what he calls the germinal vesicle, not by 

 the unwalled contents of the ovisac, which would coiTespond rather to the 

 adventitious or yolk-food of the Bird. The homologies would stand as 

 follows : — 



Vitelline memhrane wanting . 



Food yolk liquifying yolk of Huxley. 



m n-Ar ^ ^ (wall of "germinal ve- 



^- i emporary zona of Meckel j • n ,, otj -> 



True or primitive central ") ri i. x r v i 



T ^ n } Contents oi above, 



granular yolk ) 



"Wall of germinal vesicle wanting. 



Macula of do germinal spot. 



Eeasons will be given afterwards for regarding the ivall of the germinal 

 vesicle as a non-essential structure, though one certainly of great con- 

 stancy in the animal kingdom. 



E 



•f^ 



