76 



CHANGES IN THE OVUM 



cells being produced within each parent-cell. But no observer has yet 

 seen a nucleus in any of the central vesicles of the yolk segments of 

 the ovum of Mammalia. Bischoff remarks, however, that these vesicles, 



Fig. 9.* 



though not nucleated, evidently play the part of nuclei in the spherical 

 yolk segments, and that the aggregation of the yolk substance into 

 spherical segments must be due to a kind of attraction exerted by the 

 vesicles. How the multiplication of the vesicles takes place, by which 

 the subdivision of the yolk is effected, he cannot decide. But he sees 

 no positive objection to the belief that they themselves multiply by 

 di vision. \ 



Changes in the ovum subsequent to the process of cleaving, 



With respect to the changes which occur in the ova of invertebrate 

 animals after the completion of the process of cleaving, or analogous 

 processes, little need be added to what has been already said. For 



* Fig. 9, A. Ovum of a bitch, from the Fallopian tube, half an inch from its opening 

 into the uterus, shewing the zona pellucida with adherent spermatozoids, the yolk divided 

 into its first two segments, and two small granules or vesicles contained with the yolk in 

 the cavity of the zona. B. Ovum of a bitch from the lower extremity of the Fallopian tube : 

 the cells of the tunica granulosa have disappeared : the yolk is divided into four segments. 

 C. Ovum of bitch from the lower extremity of the Fallopian tube, in a later stage of the 

 division of the yolk. D. An ovum from the uterus : it is larger, the zona thicker, and the 

 segments of the yolk are very numerous. E. Ovum from the lower extremity of the Fal- 

 lopian tube burst by compression : the segments of the yolk have partly escaped, and in 

 each of them a bright spot or vesicle is visible. 



t Bischoff, Entwickelungs-geschichte des Hunde-eies, pp. 45, 46. 



