136 JOHN PRIESTLEY. 



the animal series new means of gaining the same end 

 present themselves, while the old vanish or are made use of 

 for other purposes. The phases in the act of propagation of 

 a unicellular organism may be less significant and have a far 

 different outcome in a unit-cell of a more complex creature. 



In examining the origin and growth of nuclei, Auerbach 

 chose two objects which had already been studied by various 

 previous observers, viz. the ova of Strongylus auricularis and 

 Ascaris nigrovenosa, two species of parasitic nematodes. The 

 protoplasm of these elliptical or elongated ova contains yolk- 

 granules ; and in the specimens which have already acquired 

 a yolk-membrane, a clear fluid — liquor ovi — appears to have 

 been expressed into the space immediately subjacent to it, 

 collecting in larger quantities at the extremities. For the 

 purposes of examination the ova were subjected to pressure 

 caused by the capillary adhesion of the cover-slip and glass 

 slide, a pressure which can be easily regulated by varjdng 

 the size of the cover-slip and the quantity of fluid beneath 

 it. By this means the obstruction to clear vision due to the 

 granules was to a large extent removed ; while the clear 

 peripheral layer of liquor ovi became less or entirely dis- 

 appeared. The specimens were removed from the body of 

 the nematode and mounted in iodized serum. 



If observations are commenced a short time after fertilisation 

 the ova are seen to consist merely of protoplasm and of yolk- 

 granules, the germinal vesicle having entirely disappeared. 

 Soon the yolk-granules seem to have withdrawn from the 

 periphery, leaving the latter' quite clear. Thereupon the 

 difierentiation of the future yolk-membrane begins, either as 

 a true secretory process, or perhaps, merely as a hardening of 

 the outer layer of protoplasm. The completion of the yolk- 

 membrane is a signal for the redistribution of the yolk-gran- 

 ules in the clear zone, — an act, which is, however, imme- 

 diately followed by a contraction of the whole protoplasmic 

 mass and the expression of the before-mentioned liquor ovi. 

 This terminates the formation of the primary cleavage mass 

 (PI. XI, fig. 3). 



At this point commences what Auerbach aptly calls the 

 prelude to the cleavage-drama. At two opposite points of 

 the periphery of such a primary cleavage mass — generally 

 at the poles, and immediately below the yolk-membrane, 

 clear, star-like, ill-defined spaces are seen to develope 

 (Plate XI, fig. 4). These quickly grow larger and rounder, 

 attaining a diameter of 12 — 15 fx, and their borders become 

 sharper. Meanwhile certain pale, round, sharply defined 

 nucleolar structures appear in their interior, giving them the 



