lxxxvi REPORT — 1877. 



new cells, and on which recent research has thrown a new and unexpected 

 light. 



With respect to the nature of the first segment-sphere of the ovum and 

 the source of its nucleus, as well as of the other segment-spheres or cells 

 which follow each other in the successive steps of germ-suhdivision, it appears 

 probable, from the researches of several independent observers, and more 

 especially of Edward Tan Beneden and Oscar Hertwig, that in the course of 

 the extrusion of the germinal vesicle a small portion of it remains behind 

 in the form of a minute mass of hyaline substance, to which Yan Beneden 

 has given the name of pronucleus, and that, as the residt of the fertilizing 

 process, there is formed a second similar hyaline globule or pronucleus, 

 situated near the surface, which gradually travels towards the centre and 

 unites with the first pronucleus, and that these two pronuclei, being fused 

 together, form the true nucleus of the first segment-sphere. According to 

 this view the original germinal vesicle, when it disappears or is lost to sight, 

 as described by so many embryologists, is not dissipated, but only undergoes 

 changes leading to the formation of the new and more highly endowed nucleus 

 of the first embryonic or segmental sphere. It further appears that the sub- 

 division of each segmenting mass is preceded by a change and division of 

 the nucleus, and that this division of the nucleus is accompanied by the pe- 

 culiar phenomenon of a double conical or spindle-shaped radial lineation of the 

 protoplasm, which, if we were inclined to speculate as to its nature, seems 

 almost as if it marked out the lines of molecular force acting in the organizing 

 process. These lines, however, it will be understood, if visible with the 

 microscope, even of the highest magnifying-power yet attained, belong to 

 much larger particles than those of the supposed molecules of the physicist ; 

 but, considered in connexion with what we know of the movements which 

 frequently precede the act of division of the yolk-spheres, we seem in this 

 phenomenon to have made some near approach to the observation of the 

 direction in which the molecular forces operating in organization may be 

 supposed to act*. 



With respect to the nature of the blastoderm, the organized cellular stratum 

 resulting from segmentation, and its relation to the previous condition of 



* The observations referred to above as to the division of the nucleus are so novel and 

 of such deep interest that I am tempted to add here a short abstract of their more im- 

 portant results from a very clear account given of them by Dr. John Priestley, of 

 Owens College, Manchester, in the ' Journal of Microscopical Science ' for April 1876. 



The researches now referred to are those of Auerbach, Butschli, Strasburger, Hertwig-, 

 and Edw. Van Beneden ; and the following may be stated as the points in which they 

 mainly agree : — 



The nucleus when about to divide elongates into a spindle-shaped body, becomes irregular 

 and indistinct, acquires a granular disk or zone in the plane of its equator; this divides 



