90 



GENERAL ANATOMY. 



The first changes in the ovum which take place upon conception, appear to 

 be as follows: The spermatozoon penetrates the ovum, 1 the effect of which is 

 to bring it into contact with the yelk, and with the germinal vesicle contained 

 in the yelk. It seems as if this normally occurs in the Fallopian tube, 2 and 

 abnormally it may even take place in the peritoneal cavity. The first effect is 

 to produce a cleavage and multiplication of the yelk, which becomes first cleft 



Fig. 51. 



Four diagrams to show the division of the yelk. The ovum is surrounded by spermatozoa, 

 corpuscles (polar globules of Robin) are seen in the first two. 



The clear 



into two masses, then into four, and so on, until at length a mulberry-like 

 agglomeration of nucleated cells results (Fig. 51). It appears probable that 

 this proliferation is due to some change in the germinal vesicle and its 

 nucleolus, but the nature of such change has not been made out. Some 

 observers describe it as consisting simply in the cleavage of the vesicle and 

 nucleolus, others in their disappearance and replacement by a fresh cell, or 

 nucleus, the embryo-cell, around which the yelk gathers. In this view the fer- 

 tilization of the yelk is due to the solution of the germinal vesicle under the 

 action of the spermatozoon. 



There are also found within the vitelline membrane one or more clear 

 globules, called " polar globules," by Robin, because they lie near one of the 

 poles of segmentation. The nature, origin, and uses of these bodies are not 

 known. They seem to be usually regarded as produced by the liquefaction 

 of the yelk, and as not being essential to the process of fructification. 



The globules of which the yelk is now composed soon arrange themselves 

 into the form of a membrane lined with pavement-epithelium. As the yelk- 

 mass softens, fluid accumulates in the interior of this membrane, spreading 

 it out on the internal surface of the vitelline membrane. The latter (external) 

 membrane (Fig. 53) soon becomes covered with granulations or vegetations, 

 giving it a shaggy appearance, and then takes the name of the "primitive 

 chorion," whilst the internal membrane is called the " blastodermic vesicle." 



The blastodermic membrane soon afterwards splits into two layers, the 

 division proceeding from the point where the thickening or aggregation next 

 to be described as the germinal area occurs, and extending gradually over the 

 whole circumference of the ovum, which now consists of three concentric 

 layers of membrane the external, the primitive chorion, the middle, the 

 external layer of the blastodermic vesicle, and the inner, its internal layer. 

 The annexed figure shows this division commencing. It is said that the ovum 

 is in this condition at about eight days, but no observations of the human 

 ovum at so early a period exist. The internal layer of the blastodermio mem- 

 brane next separates into two at the situation of the area genninativa. The 

 membrane which results from this separation is called the middle layer of the 

 germinal or blastodermic membrane, and is distinguished from the others in 

 not being coextensive with the embryo, but existing only at the germinal area. 



1 See Newport Phil. TrmiK. 1853, vol. ii. p. 233. This lias been since confirmed by other 

 observers on v;iri<ms lower animals, and may bo assumed to be generally true. 



2 Many physiologists, as Misehoff and Dr. M. Harry, believe that the ovum is fecundated in 

 the ovary, but the reasoning of Dr. Allen Thomson appears very c-o^ent in proving that tho 

 usual spot at which the spermatozoon meets with the ovum is in the tube. 



