OVUM. 



[87] 



more firmly united to wards the external surface. 

 This circumstance lias given rise to the belief 

 among some observers in the existence of an 

 additional delicate membrane enclosing the 

 yolk-mass ; but the most attentive observa- 

 tion by Bischoff, Wharton Jones, myself, and 

 others has failed to detect such a membrane ; 

 and there is reason to think that the con- 

 fident belief in its existence has had its origin 

 in part at least in a desire to establish a more 

 complete analogy between the ovum of birds 

 and Mammalia, and to find accordingly a 

 vitelline membrane as well as a chorion 

 present in the ova of the latter. 



The germinal vesicle is usually about a 

 sixth of the diameter of the whole ovum ; but 

 it is sometimes larger, or between a filth and a 

 fourth. It possesses a delicate membranous 

 wall of a spherical or spheroidal form and ho- 

 mogeneous structure : it is barely possible 

 to observe the double line of the thickness 

 of this wall with the quarter of an inch lens 

 in the microscope. In most animals the ger- 

 minal vesicle is readily distinguishable from 

 the rest of the ovum by its superior clear- 

 ness, excepting in those instances in which it 

 is hidden by the great opacity of the yolk- 

 substance; but then it may generally be made 

 manifest by flattening the ovum by compres- 

 sion between plates of glass. The fluid which 

 fills its cavity, which is generally very clear, 

 contains some minute granules in suspension; 

 and besides these there is apparent within it 

 the macula germinativa or germinal nucleus. 

 This last, which is in general well defined 

 in the mammiferous ovum, varies slightly 

 in different animals : in some presenting the 

 appearance of a round globule, with a deli- 

 cate circumscribing line almost amounting to 

 a vesicular covering ; but more frequently 

 it consists only of a small spherical or dis- 

 coid mass of fine granules. In a germinal 

 vesicle of ^1^" in diameter, such as that of the 

 rabbit, the diameter of the macula is about 

 one-fourth of that of the vesicle, or ^Vo"- 



In the earlier stages of formation of the 

 ovum the germinal vesicle is of smaller size ; 

 but it is then proportionally larger than the 

 other parts. It is the part of the ovum first 

 formed ; the yolk-substance, which is subse- 

 quently deposited in gradually increasing 

 quantity round it, together with the zona, 

 grow at a more rapid rate than the vesicle, 

 and thus the latter remains in the mature 

 state proportionally smaller. As the yolk- 

 substance is at first deposited nearly in 

 equal quantity on every side of the vesicle, 

 it for a time contains the vesicle in its cen- 

 tre ; but as the formation of the ovum pro- 

 ceeds the vesicle is found in general to have 

 approached the surface at one side of the 

 yolk-substance ; and in the mature ovum the 

 vesicle seems to be imbedded in the most coin- 



mined by observation in the Mammalia, nor 

 has any one as yet succeeded in observing a 

 canal or pore leading from the surface of die 

 yolk-substance towards the germinal vesicle 

 in the mammiferous ovum. 



Fig. 60*. 



Ovum of the Rabbit from the Fallopian tube with 

 spermatozoa. 



The accompanying figure is introduced to show 

 the usual position of the spermatozoa in relation 

 to the zona and albuminous layer in the ovum of 

 Mammalia daring and after impregnation. This 

 ovum is magnified '250 diameters. It was taken 

 along with five others from the lower part of the 

 Fallopian tube C8 or 70 hours after impregnation. 

 The segmentation appears to have proceeded to the 

 fifth stage. There is a thick covering of albumen 

 over the zona, and a number of spermatozoa are 

 represented involved in the albuminous substance; 

 some were also seen on the surface of the zona, and 

 some, varying in number in the different ova 

 observed from five to seven or nine, were clearly 

 ascertained to be situated within the zona on the 

 surface of and in the grooves between the yolk 

 segments. The position of these la*t is not s'uffi- 

 ciently clearly represented in the figure. 



In the situation now described the germinal 

 vesicle, though not by any means firmly fixed, 

 is yet sufficiently embraced by the yolk- 

 substance to prevent it from changing place 

 when the ovum is moved in different direc- 

 tions. In the instances of the more fluid 

 condition of the yolk it flows freely out from 

 within the zona when this has been broken ; 

 but in those ova in which the yolk-substance 

 is more viscid, as in the human ovum, we ge- 

 nerally fail to isolate the vesicle from the rest 

 of the substance. 



The macula or nucleus appears to be at- 



pact and superficial layer of fine granules of tached to the inner surface of the membrane 



the yolk-substance. This place no doubt cor- 

 responds in Mammalia, as has been ascer- 

 tained in Eatrachia, to the point from which 

 after fecundation the first cleavage of the yolk 

 proceeds; but this fact has not yet been deter- 



of the germinal vesicle. This is especially 

 seen to be the case in the Pig, in which the 

 macula seems to be somewhat pyriform or 

 pediculated (see Jig. 61*). 



No important changes have been observed 

 [G 4] 



