Dr. W. H. Eansom 07i the Stickleback, 231 



The general form of the egg after deposition is round, but it is 

 rendered irregular by indentations caused by the pressure of other 

 eggs. It is inelastic, and retains impressions made in it by a needle ; 

 and when placed in water, these characters remain for a long time if 

 it be not impregnated, — a fact which indicates that water does not 

 pass through the micropyle, or by imbibition through the chorion. 

 The viscid secretion of the oviduct which invests the eggs may 

 defend them against the action of water, in which it does not readily 

 diffuse or dissolve. This secretion has an idkaline reaction. The 

 substance of the yelk has a decidedly acid reaction, — more than 

 enough to neutralize the alkalinity of the viscid secretion. This 

 reaction is, I believe, due to a peculiar organic acid, but the ex- 

 periments relating to this question are not yet complete. The 

 seminal particles of the male continue to move for a considerable 

 period in the viscid secretion which envelopes the ripe ova, but they 

 very quickly become still in water. 



In the act of impregnation one or more (as many as four have 

 been seen) spermatozoids pass into the micropyle, and probably by 

 their proper motion overcome the obstruction which prevents the 

 entrance of water. Actively moving spermatozoids may remain in 

 contact with the chorion for eighteen minutes at least without pro- 

 ducing any sensible change in the ovum, provided none of them enter 

 the micropyle, but when one is seen to enter, in about a quarter 

 of a minute a change is observable. 



The changes which are observed to follow the entrance of the 

 spermatozoids into the micropyle are the follo\ving: — In about a 

 quarter of a minute the tube is shortened, and very soon a clear space 

 becomes visible within the chorion near the micropyle ; this space, or 

 respiratory chamber, gradually extends to the opposite pole of the egg 

 and increases in diameter, as does also the whole ovum. During 

 the formation of this space the surrounding fluid enters through the 

 micropyle, and this gradually retracts and is at length closed. This 

 entrance of fluid into the egg effaces the depressions, restores the 

 round form, and makes it firm and elastic ; but does not cause 

 any such precipitation of granular matter as is produced by its arti- 

 ficial introduction. 



While the respiratory chamber is yet in progress of formation, the 

 yellow drops of the superficial layer of the yelk grow pale and 

 disappear ; the change beginning near the micropyle. As a result 

 of this, the whole egg becomes clearer, and the discus proligerns, 

 which may be now more correctly denominated the germinal mass, 

 is more distinct. 



The yelk now very slowly alters its form, one surface becoming 

 flattened ; but about fifteen or twenty minutes after impregnation a 

 remarkable and more vivid contraction begins, causing the yelk to 

 pass through a series of regularly recurring forms. The contraction 

 begins on one side near the equator, and soon forms a circular con- 

 striction which gives the yelk the figure of a dumb-bell, the longer 

 axis of which is the polar axis of the egg. The constriction travels 

 towards the germinal pole, and next produces a flask-shaped figure ; 



