16 COMMISSION OF CONSERVATION 



somes of both egg and sperm nuclei. To follow these processes requires 

 the employment of highly technical methods and turns the attention from 

 the more superficial phenomena of gross structures to a penetration into 

 the more profound constitution and intrinsic activities of living proto- 

 plasmic units. An egg-cell contains so many minute parts and undergoes 

 such rapid internal changes that it is impossible to watch all that goes on. 

 The aggregation of yolk-granules in the protoplasm especially obscures the 

 vision and even materially interferes with or modifies the processes them- 

 selves. These difficulties are largely overcome by making great numbers 

 of observations on living eggs, some of which are more favourable than 

 others; by preserving eggs with chemical reagents at various stages and 

 then using different agents to stain and to clear until otherwise invisible 

 parts are rendered apparent; by comparison with more transparent eggs 

 of other animals; and by resorting to a method of sectioning of eggs large 

 enough to permit it. 



There are in the protoplasm of the egg at this period two bodies of 

 such special activity as to be considered of commanding importance, the 

 original nucleus of the egg and the transported head of the spermatozoon. 

 Each begins separately a series of changes preparatory to their union. 

 Those changes which belong especially to the egg and its nucleus are re- 

 garded as a process of maturation. A mass of clear protoplasm about the 

 nucleus separates into two portions (astrospheres) which become star-like 

 and moved to opposite poles of the nucleus (Plate V, fig. 6). At the same 

 time the nuclear membrane and nucleolus disappear, leaving the chromo- 

 somes free. Rays of the astrospheres diverge in all directions from two 

 central bodies (centrosomes), some of them extending outwards through 

 the protoplasm to the ectoplasm, others stretching inwards to become 

 attached to chromosomes. Contraction waves pass over the surface; the 

 egg-membrane becomes wrinlcled as if the ectoplasm were being pulled 

 away from it; a quivering motion of the protoplasm is observable; the 

 astrospheres and chromosomes are carried towards the pole of that hemi- 

 sphere of the egg which becomes the larger. Those rays which lie along 

 the axis between the two centrosomes form a nuclear spindle, and by con- 

 traction the chromosomes are divided into two groups. One of these 

 groups is thrust outwards, carrying with it portions of an astrosphere, 

 protoplasm and egg-membrane, which constricts around and separates 

 the mass as a small globule (first polar globule) on the surface of the rela- 

 tively large egg (fig. 7). A nuclear spindle is reconstructed within the egg, 

 the chromosomes again divided, and a second polar body soon formed 

 at the base of the first (fig. 8). In the eggs of some animals it has been 

 observed that the chromosomes become split longitudinally into double 

 their number during the formation of the first polar body, and that this 

 polar body di\ades, each part taking half the chromosomes during the 

 formation of the second polar body. This would seem to show that the 



