146 JOHN PRIESTLEY. 



{pronucleus engendre). The latter is the remnant of the 

 clear substance which had accumulated at the poles of the 

 spindle-shaped nucleus in the primary cleavage mass, and, 

 being merely differentiated protoplasm of the cell-body, it 

 has no genetic connection with that nucleus. The derived 

 pronucleus increases at the expense of the produced pro- 

 nucleus until the latter is all absorbed, when the former ac- 

 quires nucleoli and constitutes the nucleus of the segment. 



At this point van Beneden's research ceases to have 

 special interest for us now ; a detailed account of what 

 follows need not, therefore, be given. To be brief, certain 

 differences in size, microchemical reaction, and subsequent 

 development presented by the cleavage segments of the 

 second order, led him to apply the terms ectodermic and 

 endodermic to the larger and smaller masses respectively. Of 

 the masses of subsequent orders, those derived from the 

 ectodermic segment divide more rapidly, and in such a 

 manner as to envelope as a cortex those derived from the 

 endodermic segment. The cortex is provided with an aper- 

 ture, called by Ray Lankester a blastopore, through which 

 is extruded an endodermic stopper ; and the whole structure 

 is designated Metagastrula. 



In conclusion, van Beneden, after describing the formation 

 of the blastodermic vesicle (germ-vesicle), gives a brief 

 resume of the results of his researches on the multiplication 

 of the cells of the ectoderm in rabbits — results which are 

 almost identical with those of Strasburger. ^ The elongating 

 nucleus becomes indistinct and irregular in margin. The 

 nucleoli disappear ; the contents separate into a clear sue nu- 

 cleaire, indifferent to staining fluids, which collects at the 

 extremities, and a homogeneous essence nucleaire, tinging 

 deeply in carmine, which collects in the middle as an 

 equatorial plate or disc of highly refringent, ovoid granules. 

 At this stage, it is to be remarked, Beneden never saw the 

 longitudinal striation remarked by Strasburger and others. 



A finely granular mass now collects about the poles of the 

 nucleus — a mass Avhich is possibly the same as that of the 

 pronucleus engendre spoken of before ; and the protoplasm 

 of the cell presents the appearance of stars, indicating 

 clearly an attraction exerted by the nuclear poles. The 

 equatorial plate splits into two discs which move apart, 

 beint' merely connected by a few threads stretched between 

 their opposing faces. After a time, however, the threads 

 are retracted into the discs. 



The discs reach the poles and finally become the young 

 nuclei. The intermediate clear tract differentiates at its 



i 



