HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 11 



Fecundation not only accelerates and intensifies the polar aggregation 

 of the protoplasm, but also modifies the form of the disc and apparently 

 its character. Our observations in this direction are mainly of a casual 

 nature, but, so far as they go, they confirm the testimony of other writers. 

 Ryder's 4 studies on the eggs of the coil and the shad are of interest in 

 this connection. 



Facts of this nature show how closely inter-related are the processes go- 

 ing on in the egg, and how difficult it is to analyze and classify them The 

 various factors at play arc not independent actors; and in describing the phe- 

 nomena it is well to avoid any classification that, presupposes the contrary. 



We will first describe the formation of the germinal disc in surface 

 views and optical sections, and then examine the conditions shown in 

 actual sections and in discs mounted in Mo. 



A. Formation of the Blastodisc. 



Plates XX. -XXIII., XXVI. 



1. The Ripe Egg. — The ripe, unfertilized egg of Ctenolabrus. like 

 those of other species that have come under our observation, is more or 

 less opaque, according to the relative abundance of the minute refractive 

 granules lodged in the peripheral layer of protoplasm (Fig. 1, PI. XX.). 

 In eggs taken from the fish by gentle pressure, one meets with different 

 degrees of opacity, and our experience leads us to conclude that the 

 more opaque eggs are less advanced in maturity. When the eggs flow 

 freely under most careful handling, and may therefore be supposed to 

 have reached a high state of maturity, they are only partially opaque, 

 and clear up on touching the water with most astonishing rapidity. On 

 the other hand, if considerable pressure is required in the operation, they 

 are usually found to be more opaque, slower to clear up, and more liable 

 to escape fertilization. 



In some instances we have seen the fertilized eggs clear up within five 

 seconds, and the process was only a little less rapid and complete in those 

 left unfertilized. We have endeavored to ascertain by direct observation 

 what becomes of these granules. They appear to swell a little, and dissolve 

 in situ; at least, we have seen no evidence of coalescence, and are quite 

 positive that they are not discharged into the perivitelline space, as con- 



* John A. Ryder. A Contribution to the Embryography of Osseous Fishes. United stales Commis 

 sion of Fish and Fisheries. Report of the Commissioner for 1882, pp. 10, 11. Washington, 1884. 



