940 SPECIAL PHY.SIOLOGY. 



in the development of the embryos of all animals, even of the highest 

 Vertebrata ; but these are oftentimes rapid, and occur in such an 

 early stage of embryonic life, as not to be so obvious. 



Ova and Pseudova. A true ovum, the product of a female organ 

 or ovary, is a nucleated cell, possessing a delicate cell wall, a contained 

 nucleus, within which is a nucleolus, arid, besides that, certain cell 

 contents. It is a proper and special germ-cell, set apart for the re- 

 production of a new individual. 



The male product, or fertilizing element, the product of the so- 

 called sperm-cells, formed in the testes, is a fluid containing micro- 

 scopic bodies named spermatozoa; these are endowed with the power 

 of active movement, which lasts, in the Warm-blooded Vertebrata, for 

 a few minutes, in the Cold-blooded Fishes, for days, and in certain 

 Mollusca and Annulosa, even for months, when received into the spe- 

 cial receptacle, or spermotheca. From their mode of development, 

 from the character of their movements, and from the effects of re- 

 agents upon them, they may be regarded as ciliated gymnoplasts, or 

 ciliated nuclei, which may be compared to single particles of ciliated 

 epithelium. The sperm-cell in which they have their origin, and from 

 which they escape by rupture of the cell- wall, is the homologue of the 

 germ-cell, or ovum. 



In true sexual reproduction, the product of the sperm-cell enters 

 and fertilizes the germ-cell, and imparts to it the power of specific re- 

 production, just as the pollen of the anther of a flowering plant fer- 

 tilizes the vegetable ovule. 



The unfertilized ovum of a queen or female bee, and also the pseud- 

 ova of the Aphis, and of other animals propagated by alternate 

 generation, are also nucleated cells, portions of the parent animal, set 

 apart for particular purposes, and retaining special powers of further 

 evolution; they are, therefore, also germ-cells, or rather germinal cells. 

 They may be viewed as undeveloped, or ametamorphosed portions of a 

 previously fertilized blastema, which has itself resulted from the first 

 stages of evolution of a true ovum ; they are, however, retained in 

 connection with some portion, usually internal, of the non-sexual and 

 only indirectly fertilized offspring, waiting for their opportunity of in- 

 dividual evolution. They have, in truth, been fertilized. According 

 to this view, every individual animal form, whether the result of direct 

 sexual evolution, or of parthenogenesis, or of any stage of alternate 

 generation, is produced from a primitive cell, which, having been di- 

 rectly or indirectly fertilized, undergoes multiplication and differen- 

 tiation, so as to evolve the future animal. The simplest forms of 

 reproduction, by gemmation or by cleavage, are but extensions of in- 

 dividual animals, themselves traceable to the evolution of two primi- 

 tive, sexually developed, fertilizing, and fertilized cells. Even in the 

 lowest Protozoa, evolutions of new beings, from time to time, occur 

 by the conjugation of two nuclear particles in their interior, which, at 

 least, imitate a sexual process. 



Whatever variety the reproductive process of animals may present, 

 the primitive cell, whether it be & fertilized ovum, an unfertilized ovum, 

 a pseudovum, or the commencement of a bud, is, in all known cases, 



