INTRODUCTION. xlv 



are perforated by one or more minute openings which serve for the passage into the 

 egg of nutritive material from the jjareat, and for the introduction of water in aquatic 

 forms. Besides, in many forms there is a lai-ger opening, the microjiyle, for the entrance 

 of the spermatozoon which is to fertilize the egg. 



Kecent investigations have shown that an egg or a cell is far from the simple 

 structure which it was once imagined to be ; the protoplasm of the cell is not a homo- 

 geneous substance, while the nucleus or germinative vesicle is very complex. The latter 

 is enveloped by a special membrane and filled with a protoplasm, in which floats a tan- 

 gled network of fibres. Wliat is called the nucleolus is now regarded by Flemming 

 and by Carnoy (two of the most profound students of cells) as a specialized portion or 

 portions of the network. The nucleolus (there may be three or more in an egg) is 

 called in the older works tlie germinative spot, or the Wagnerian vesicle, the latter 

 name being a]iplied in honor of its first discoverer. In the living egg the nucleolus 

 is usually readily distinguished under the microscojie by its great refraugibility, but to 

 recognize the network it is necessary to employ stains and other reagents. 



This egg, as we have described it, undergoes an extensive and complicated series 

 of changes (known as the maturation of the egg) before it is ready for impregnation, 

 although it is to be noted that in some instances the maturation is concomitant 

 with impregnation. These changes may be summarized as follows: — At first the 

 nucleus occupies a position near (but rarely at) the centre of the egg ; it now moves to 

 near the surface, where its membrane breaks down, and the filaments, etc., almost en- 

 tirely disappear or at least lose their former character. In the place where the last 

 renniants of the nucleus were seen, there now appears a spindle-shaped body made up 

 of granules arranged in lines, while from either end other 

 lines of granules are arranged in a radial manner. The 

 whole ])resents an appearance closely similar to that seen 

 when iron filings are exposed to the influence of a horse- 

 shoe magnet, while from its resemblance to two stars joined 

 it has received the name amphiaster. It may be observed 

 in iiassing, that amijhiasters are cliaracteristic not onlv of ^'^ ,^/ —Formation of polar 



I !^T I J globule n nucleus ]j polar 



the maturation of the egg, but of cell division as well : giobuie » spmdie shaped 



o3' 7 bgure 



tlie connection between the two will appear in the sequel. 



The maturation spindle usually takes a position at nearly right angles to the surface of 

 the egg, and soon from the outer end a prominence appears, extending out beyond the 

 rest of the egg. The spindle now divides, and the prominence separates from the egg 

 and forms what is known as a polar globule. Again the portion of the spindle which 

 remains within the egg approaches the surface and a second polar globule is formed in 

 the same manner as the first. Now, the part of the spindle left in the egg assumes a 

 nearly spherical condition, and sinks back into the egg, where it appears exactly like 

 the original nucleus. It is called the female pronucleus. 



The meaning of these wonderful phenomejia is far from evident. The best expla- 

 nation as yet advanced, is that givfen by Balfour and Minot independently, which 

 gains additional plausibility from the fact that essentially similar phenomena are seen 

 in tlie formation of the male reproductive elements, the spermatozoa. In brief it is 

 this : — All cells have inherited from their protozoan ancestors the elements of both 

 sexes ; they are hermajihroditic, and the eggs and spermatozoa cells are the same. Be- 

 fore they can unite it is necessary that each should get rid of the element to be sup- 

 plied by the other, and in this light the formation of the polar globules is to be viewed 



