MACKEREL INVESTIGATIONS IN 1897. 7 



less numerous perforations exist in all parts of the vitelline membrane, 

 but are difficult to demonstrate uutil during the later stages of devel- 

 opment, when the membrane becomes more brittle and the perforations 

 perhaps larger. 



The egg proper, or vitellus, consists of a vesicle-like skin of proto- 

 plasm, faintly granular, of a pale brown color and of uniform thickness 

 throughout, distended by a perfectly fluid transparent yolk, in which 

 there are no structural elements observable, except occasionally a few 

 aggregations of spherical bodies attached here and there to the inner 

 surface of the protoplasmic pellicle. Floating freely about within the 

 protoplasmic skin and, because of its lower specific gravity, always 

 rising to the highest point in the egg, is a beautifully clear and per- 

 fectly spherical oil-drop, by virtue of the presence of which the egg is 

 buoyant. The oil-drop also is enveloped by a delicate protoplasmic 

 sheath, which is at first of uniform thickness and always quite distinct 

 from the protoplasm enveloping the yolk. The oil-drop is of unusually 

 large size, but varies both absolutely and relatively to the size of the 

 egg to a considerable degree. Its average size is 0.28 mm. Its color 

 also varies. Sometimes it is perfectly colorless; sometimes, and more 

 frequently, of a pale amber, or even of a pinkish tint. And this varia- 

 tion maybe observed in eggs taken from the same female. The reason 

 for this is not apparent, but it was noticed that colorless oil-drops occur 

 most frequently in eggs of the smaller sizes. Perhaps both may indi- 

 cate a condition of impaired vitality, though this could not be proved 

 by the results of the further course of development. 



Owing to the unfavorable circumstances under which the mackerel 

 eggs were taken, I was unable to study the maturation processes in 

 the living egg, but material was preserved for a future study of this 

 phase of development. There is no reason to doubt, however, that the 

 process is essentially similar in this and other pelagic eggs, and I refer 

 to my report on the Spanish mackerel and to the papers of Agassiz 

 and Whitman on the dinner, and of Ryder on the cod, for some obser- 

 vations on these phenomena. The process consists essentially in the 

 elimination from the egg mother-cell of a part of its nuclear substance, 

 without which it seems incapable of fertilization. Accomplishing this 

 by two successive nuclear divisions, the surplus material is cast out 

 within two minute cells, the so-called polar bodies, which appear at the 

 micropylar pole of the egg within a few moments after it reaches the 

 water, pass into the peri vitelline space, and lose further significance in 

 development. The maturation processes appear to be hastened by 

 copulation (the contact of a spermatozoan, after entrance through the 

 micropyle, with the vitellus), and indeed are largely coincident with 

 this, but may take place quite independently of it, in which case the 

 time required for the completion of the process is longer. Not until 

 after the extrusion of the polar bodies is the egg mature and ready for 

 the union of the egg, or female pronucleus, and the sperm, or male 

 pronucleus, which constitutes the essential act of impregnation. When 

 this union takes place there is constituted a cell fundamentally different 



