62 



RECENT ADVANCES 



PHYSIOLOGY OF DEVELOPMENT. 



OF THE CHANGES IN THE OVUM PREVIOUS TO THE FORMATION 

 OF THE EMBRYO.* 



UNDER this head, it is proposed to bring together all the new facts 

 relating to the subject, which have been derived from observations made 

 on the invertebrate, as well as on the different classes of vertebrate, 

 animals. 



The most important subject for consideration, will be the division and 

 subdivision of the yolk. Before entering upon this topic, however, it is 

 necessary to inquire whether the ovum does not, immediately before, or 

 immediately after, its extrusion from the ovary, undergo other changes 

 previous to the manifestation of the remarkable phenomenon of spontaneous 

 cleaving. 



Changes in the germinal vesicle and germinal spot. Dr. Barry stated, 

 that, previous to the discharge of the ovarian ovum, the germinal 

 spot returns to the centre of the germinal vesicle, and the germinal 

 vesicle to the centre of the ovum ; and that the germinal vesicle is not 

 dissolved, but that while it and the germinal spot undergo the changes of 

 place just mentioned, a peculiar process of cellular development occurs, 

 which ends in the formation of two cells in the centre of the yolk, which 

 have an important destination in reference to the formation of the 

 embryo. f With reference to these various points, however, both M. 

 Bischoff and Mr. Wharton Jones think that Dr. Barry's statements are 

 erroneous. Bischoff describes the yolk of an ovarian ovum after coitus, as 

 being unchanged in its characters, with the single exception of being fuller 

 and more dense : it is still granular as before, and does not possess the 

 layers of nucleated cells described by Barry. He thinks also, and in this 

 opinion he is supported by Mr Wharton Jones, that the movement of the 

 germinal vesicle and germinal spot from the surface to the centre of the 

 ovum, could not possibly be observed even if they took place. Moreover, 

 he is led by his observations to the conclusion that, contrary to Dr. 



* Book the Eighth, section i. p. 1508 of M tiller's Physiology. 



t Third Series of Researches in Embryology in Philosoph. Transact. 1840. 



