ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 567 



is not of a pole of the spindle, but takes place in the equatorial 

 plane." 



In the vitellus a homogeneous permtelline layer is differentiated 

 peripherally, and the refringent body in the caudal portion of the 

 zoosperm is ejected into the perivitelline space. In spite of the 

 striking analogies, the genesis of the first polar body is not to be 

 compared to indirect cell-division. 



After the elimination of the first polar body there remains an 

 homologous body, the deuthyalosome. This latter body increases in 

 size at the expense of the vitellus and developes two " asters " on its 

 surface, one peripheral, the other central. Complicated pseudo-Jmryo- 

 Mnetic figures are now formed, but disappear before the elimination of 

 the second polar body, which is formed from the deuthyalosome much 

 in the same way as the first polar body from the prothyalosome, and 

 apparently at the same spot on the ovum. A second perivitelline 

 layer is now formed. The second polar body (i.) is the equivalent of 

 the female pronucleus which remains behind, and (ii.) cannot be 

 regarded as a cell. 



So far, Van Beneden concludes, no phenomena of true fertilization 

 have occurred, merely " phenomena of the maturation of the ovum." 



The formation of the pronuclei and the true phenomena of fertili- 

 zation are treated in the fourth chapter. The female pronucleus (the 

 equivalent of the second polar body) consists of chromatic and 

 achromatic elements, derived from the germinal corpuscle and the 

 prothyalosome respectively. Contemporaneously with the expulsion 

 of the second polar body, the male pronucleus is formed, exclusively 

 from the nucleus of the zoosperm. Ultimately the two similar pro- 

 nuclei meet in the centre of the reduced ovum (or female gonocyte, as 

 it is now termed), and unite partially without fusion. A single 

 dicentric karyokinetic figure is now formed (derived equally from the 

 two pronuclei), and segmentation begins. 



"The egg, furnished with its two pronuclei, behaves like a single 

 cell, and the sum of the tioo nuclear elements is equivalent to a 

 simple nucleus. The first cell of the embryo is accordingly formed 

 from the moment when the two pronuclei are fully developed ; fertili- 

 zation coincides with the genesis of the two pronuclei." 



Van Beneden concludes that " fertilization consists essentially in 

 the formation of the female gonocyte, and its transformation into a 

 cell, that is to say, in the replacement of the expelled elements by 

 the now elements introduced by the zoosperm. The polar bodies are 

 replaced by the male pronucleus." 



All cells of the tissues are thus hermaphrodite, and fertilization 

 is not a generation but merely a substitution requisite for the in- 

 definite conservation of life. 



Spermatogenesis in Ascaris megalocephala.* — Wo have yet 

 another contribution to our knowledge of tlie development of sjierma- 

 tozoa, from E. van Beneden and C. Julin. The Nematodes in general 

 and Ascaris megalocephala in particular lend themselves remark- 



* Bull. Acwl. R. Sci. Belg., vii. (1884) pp. 312-42, 



