24 HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 



minable, but it is safe to say it is not less than twenty nor more than 

 twenty-six. We have not attempted to trace the origin of the second 

 spindle, and we can only say that it presents the same appearance as 

 the first. 



y. The Male Pronucleus. — It is not until about ten minutes after fer- 

 tilization that the spermatic element clearly reveals itself as a nuclear 

 body. It appears at this time (Fig. 13, PI. XXII.) as a minute chromatic 

 corpuscle in a clear area, which is the centre of distinct astral rays, 

 and at the same Urn* 1 flic centre of the blastodisc. This astral figure lies at 

 first nearer the outer than the inner surface of the disc, but soon sinks 

 to, or below, the middle of its vertical axis. The chromatic corpuscle 

 appears to be homogeneous, but it is very small — not over 4 or 5 jx in 

 length — and inaccessible to high powers in surface preparations ; hence 

 we cannot affirm positively that it is not already divided into what Biihm 

 calls spermatomerites, Platner cart/osomes. Ten minutes later, however, we 

 do find this division carried out (Figs. 14, 15). We have not been able 

 to learn how many merites are formed, but our drawings show a rounded 

 mass of four or five parts, and there cannot be the least doubt about 

 the identity of these elements with the spermatomerites of Petroinyzon. 

 Biihm finds four primary merites in Petrom vzon, and cites Kupffer !T as 

 having represented five in the trout. Platner 18 recognized two in Avion 

 empiricorum. 



Bbhm's observations on the egg of Petromyzon and Boveri's 19 (No. 13 

 and No. 19) on the nematode and the echinoderin egg, make it quite 

 certain that this spermatic aster gives rise to the two primary cleavage 

 asters. We have not seen any acromatic corpuscle, such as Boveri has 

 described under the name '• centrosoma " ('• corpuscule polaire " of Van 

 Beneden and Neyt 20 ). 



In our preliminary paper (No. 2, p. 44) the male pronucleus is repre- 

 sented as lying above the female pronucleus. This error was corrected 

 in our first memoir (No. 1, p. 19), and we have now brought the proof 

 there promised, that '■'•the mule pronucleus becomes the centre around which the 



17 C. Kupffer. Die Befruchtung des Forelleneies. Bayer. Fischereizeitimg, 18S6. 



18 G. Platner. Ueber die Befruchtung bei Arion empincorum. Arch. f. mikr. Anat., XXVII., 1SS6, 

 p. 54. 



19 Thcodor Boveri. Ueber Partielle Befruchtung. Sitz -Ber. d. Gesell. f. Morph. und Physiol, zu 

 Miinclien, IV., 1888. 



20 Ed. Van Beneden and Neyt. Nouvelles Recherches sur la Fecundation et la Division Mitosique chez 

 l'Ascaride nicgalocephale, 1S87. Le Moniteur Beige et Bulletin de l'Acad. Roy. de Belgique. 



