70 



LUCE K N A II 1 .E ASD T U E 1 K ALLIES. 



CHAPTER VI. 



EMBRYOLOGY. 



§ 19. The Egg and (he Spermatozoa. 



140. The Egg. (PI. ix, Jig. 108).— If it be taken for granted that all the 

 illustrations of the eggs of diverse animals are full and faithful representations of 

 their structure, then the one wliicli we now offer for the consideration of naturalists 

 is fully as comphcated as any that exists. This, however, we are not at all willing 

 to believe, for we have had such glimpses of what may be learned by tlie use 

 of the microscope of the present day, as lead us to avow emphatically, and Avith- 

 out fear of disproof hereafter, that the structure of the egg, as described and 

 illustrated in various works, is "useless, except in the most general way. Wliat we 

 want is an exhaustive comparative study of the development and anatomy of the 

 ovum from its inception in the cytohlastevton of the ovary, through all its phases, 

 up to the period when it is fecundated and passes on to its second stage. Such a 

 series, for even any one species, does not exist in all our Avorks and papers upon 

 embryology. The labor is too hard to tempt any one of ordinary strength and 

 patience. The single figure on our plate represents a fully, developed egg of* 

 Halidijstus avricula, as seen in profile. It was difficult to persuade ourselves, at 

 first, that the egg of so lowly an animal could be so highly organized ; we suspected 

 some mistake; yet repeated observations only led to the same result; and we there- 

 fore present it here as an isolated fact, but with the remark that, as far as it goes, 

 i. e., as a representation of the consummation of one stage in the life of the animal, 

 it is as full in detail and physiognomy as the best lenses of the time enabled us to 

 make out. Tlie albuminous and oleaginous poles are segregated with remarkable 

 distinctness. The former or germinal vesicle (gv) occupies a pretty large proportion 

 of the space embraced by the vitelline membrane, and lies close to the inner sur- 

 face of the latter. The rest of the mass consists of the oleaginous material or 

 viiellns (vi). 



141. The Viiellus. — The form of the eg" is an irrejjular oval, and its mass is 

 more or less flattened by mutual pressure arising from the crowded state . of these 

 bodies in the saccule. It measures about yi^ of an inch in length. As many as 

 three have been found in one of the pouches of a saccule. The vitelline sac (vs) is 

 very thin, but is easily distinguished from the surface of the yolk on account of the 

 peculiar structure of the latter, and presents sharp' smooth exterior and interior 

 surfaces. Its thickness is about ^5^ o o of an inch, and it appears to be homogeneous 

 in^ texture throughout— a perfectly structureless, colorless membrane. The yolh 

 (vt) has all the appearance of passing through the process of segmentation, but the 

 presence of the sharply defined germinal vesicle and macula warns us against 



