14^ JOHN PRIESTLEY. 



vesicle undergoes a regressive metamorphosis, appearing first 

 at the periphery of the ovum as a lenticular body of punctate 

 mass, and embedding, in addition to the germinal spot, certain 

 larger irregular structures, which do not stainreadily in carmine. 



After a time the germinal spot is no longer visible in the 

 degenerate vesicle ; b.ut, instead, there is seen in the cell- 

 body a clear round structure with no membrane, but Avith 

 the capability of deeply tinging when treated with carmine. 

 This body, which Hertwig calls the nucleus of the ovum 

 [Eikern), never appears while the germinal spot is yet within 

 the vesicle, from which circumstance, in addition to its 

 deportment towards staining fluids, it is supposed to be 

 the escaped germinal spot. Finally, the vesicle becomes 

 entirely lost to view. 



After fertilisation has taken place a small clear spot 

 appears at the periphery of the ovum below the yolk-mem- 

 brane, about which as a centre the yolk-granules commence 

 to arrange themselves in radiate lines. Within the light 

 space a small dark spot is seen having a like reaction to 

 carmine with the nucleus of the ovum. From subsequent 

 phenomena, and from the fact that he occasionally saw a 

 delicate streak extending from the clear spot towards the 

 periphery of the ovum, Hertwig believes the spot to be the 

 head or nucleus of the fertilising spermatozoid, and accord- 

 ingly he has named it the nucleus of the sperm [Spermakern) . 

 The two nuclei (the pronuclei of other observers) approach 

 and meet near the centre of the ovum, where, after the nucleus 

 of the ovum has exhibited certain variations of outline, due 

 possibly to amoeboid movement, they fuse together, while 

 the dark central spot of the sperm-nucleus disappears from 

 view. The resulting structure is called the nucleus of the 

 first cleavage mass or first cleavage nucleus. About it 

 a clear substance gradually collects, giving its outlines a 

 faintness or indecision, while itself elongates into a spindle. 

 The collecting clear substance aggregates chiefly about the 

 tips of the spindle, towards which also the radiate strise of 

 the cell-body converge in two sets. 



There is, in fact, seen the bistellate figure characterised by 

 Auerbach as karyolytic. The stars increase in size, the rays 

 extending to the circumference of the ovum ; and the nucleus 

 between them, suddenly becoming indistinct, seems finally to 

 disappear. 



Division of the cell-substance now follows in the manner 

 already described, being preceded by amoeboid movement, a 

 circumstance which Auerbach has also mentioned. The 

 young nuclei seem to originate as described by the latter 



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