HISTORY OF THE EGG FEOM FERTILIZATION TO CLEAVAGE. 19 



the surface of contact enlarges, the nuclei flattening against each other 

 (Fig. 4 a). At the end of seven minutes they present the form seen in 

 Fig. 4 b, the line of junction between the two hemispheres still remaining 

 quite distinct. A moment later this line becomes obscure, first at the mid- 

 dle, then at the ends; and eleven minutes after first contact the coalescence 

 is complete, and we have a perfect sphere 20 fj, in diameter. The cleavage 

 nucleus thus formed vanishes within one minute, passing into the amphi- 

 astral phase. 



During the period of conjugation the nuclei lie in a clear area, indicated 

 by the dotted line in Fig. 4 a-b. The same area reappears in correspond- 

 ing phases of the cleavage nuclei, and the substance contained in it is 

 undoubtedly identical with the j>rri]>h(4 of Vejdovsky 12 and the archoplasm of 

 Boveri. 13 Does it not also correspond to the pole-plasm, which Bohm 14 finds 

 in the egg of Petromvzon, and which he derives from the germinal vesicle? 



Bohm has a very strong case, but we cannot grant that the origin of 

 the pole-plasm from the germinal vesicle is conclusively established. What 

 the origin of this substance in the fish egg is, we have not been able to 

 determine. The question must be decided on more favorable material. 



From the above measurements it is evident that the pronuclei increase 

 somewhat in volume during conjugation. In our preliminary paper (No. 

 2, p. 44), the deeper pronucleus was supposed to be the egg nucleus. We 

 have since been aide to trace its origin to the spermatic element. 



2. Views obtained from Mounted Preparations. 



a. Method. — In eggs killed before the blastodisc has formed, it is not 

 an easy matter to find the germinal pole. We have found but one way to 

 accomplish this ; namely, staining with osmic acid, and following with Mer- 

 kel's fluid, as before described. This method of preparation leaves the 

 germinal pole darker than the rest of the cortical layer; and, thus marked, 

 the pole can usually be found under the dissecting microscope. Having suc- 

 ceeded in this, we break the egg by the aid of needles, and remove the yolk 

 and most of the outlying cortex. Frequently it has been necessary to re- 

 move the yolk first, and then search the periblastic envelope over for the 

 pole by transmitted light. The first thing to be looked for in the mounted 



12 Franz Vejdovsky. Enhvickcliinirstrosrliiolitlirlie Untersuchungen. Heft L, l'rng, 1888. 

 18 Theodor Boveri. Zellen Studien. Jenaische Zeitsohrift, XXII., |>|>. :$. 1, 1S88. 

 14 A. A. Bohm. Ueber Reifung and Befrachtung des Eies von Petromyzon Planeri. Arch. f. raikr. 

 Anal., XX XII., 1888. 



