208 



MR GEORGE BROOK ON THE 



indentations of the surface of the vitelline mass as in the herring ovum. Out- 

 side the yolk the germinal protoplasm is collected into a distinct layer, usually 

 of a pale yellow tint. This layer is always sharply marked off from the yolk 

 at the time the ovum leaves the oviduct. As the egg floats in the water the 

 germinal layer collects at the lower pole, leaving only a delicate film of proto- 

 plasm around the upper portion of the yolk sphere. Segmentation then 

 commences if the egg has been fertilised. In the herring, however, the changes 

 are somewhat different. At a period varying from fifteen to thirty minutes 

 after the introduction of spermatozoa to sea water containing ripe eggs, the egg 

 membrane begins to leave the yolk, and by the time an hour has elapsed the 

 membrane has expanded to such an extent that a large cavity is formed all 

 around the vitelline mass. This cavity is filled with water, which probably 

 contains a portion of the egg contents in solution. The accompanying table 

 shows the increase in the size of the egg after inception of water. The first 

 two items are the measurements given by Kupffer, and may be taken as a 

 type of the eggs of the Baltic herring. The others are measurements of the 

 eggs of herring taken from the Ballantrae Banks, off the Ayrshire coast. 



Diameter 



of 



unfertilised 



oito' 



of*" 



•92 mm. 

 1 - mm. 



1-194 mm. 



average 

 1-1759 mm. 



Diameter 

 of yolk 



after 

 1 hour. 



1-147 mm. 



Diameter of 



egg capsule 



after 



1 hour. 



T486 mm. 



Diameter when breathing chamber is complete. 



Yolk. 



Greatest 

 diameter. 



•85 mm. 

 •97 mm. 



1-298 mm. 



Diameter 

 at right 

 angles. 



•82 mm. 

 •92 mm. 



1-204 mm. 



Egg capsule. 



Greatest 

 diameter. 



1"2 mm. 



1-29 mm. 



1-768 mm. 

 (after 5 hours) 

 1-599 mm. 

 1-599 mm. 

 1-599 mm. 

 1-542 mm. 



Diameter at 

 right angles. 



(after 45 rnin.) 



) 



165 mm. 

 (after 5 hrs.) 

 1-580 mm. 

 1-486 mm. 

 1-448 mm. 

 1-430 mm. 



The eggs with which Kupffer experimented were smaller than those from 

 Ballantrae, but even taking this into account, it will be seen that the latter 

 have proportionately a considerably greater diameter after the breathing chamber 

 is formed. Kupffer's measurements were made forty-five minutes after fer- 

 tilisation, and he does not state whether any further increase in the diameter 

 of the egg capsule was noticed at a later period. In the measurements which are 

 given for comparison, it will be seen that in example No. 3 the egg capsule 

 measured 1*486 mm. one hour after fertilisation. At this stage both yolk and 

 egg capsule were comparatively globular. At this stage the germinal proto- 

 plasm had collected to a considerable extent to the surface of the yolk, but was 



