68 GENERAL SKETCH OF 



various shapes in different fishes (Fig. 19). Fluid is readily 

 transmitted through this entrance, and also by the pores as 

 may be seen by partially drying the egg, and then immersing 

 it in sea water a feature connected with the aeration of the 

 contents. 



Within the capsule we have the globe of more or less fluid 

 yolk or nutritive material surrounded by a delicate layer 

 of protoplasm, between which and the capsule is a space 

 termed the perivitelline space. The entire egg of the cod 

 or whiting (Fig. 16) is a transparent spherical floating mass 



FIG. 16. Egg of Whiting, with cap of protoplasm. E. E. P. 



delicately sensitive to every movement in the water, even a 

 breath in a still vessel setting the almost invisible spheres in 

 commotion; so that under the microscope even in a quiet room, 

 and still more in a wooden building, we may understand it 

 is not always easy to keep them in the field of vision. 



In the spawning-areas, at the period of the shedding of these 

 eggs, the water swarms with the minute sperms of the male, 

 one of which, in the course of events, finds entrance by the 

 aperture (micropyle Fig. 19 a) in the wall of the egg, and 

 then begin those changes which mark fertilisation, such as the 

 fusion of the male with the female pronucleus, the protrusion of 

 the polar globules and the formation of the first segmentation- 

 nucleus. 



Shortly after fertilisation the egg in many cases becomes 

 more tense and the space within the capsule, the "breathing 

 chamber" of Needham, more evident. 



In the cod and others, moreover, a streaming movement 

 of the jelly-like protoplasmic covering of the yolk downwards, 



