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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 proligerus, 

 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 ; 

 this is at length lost by the constriction passing on, and the round 

 form is regained in about a minute. This wave reappears and 

 travels forward again without any distinct period of rest, and I have 

 seen these movements continue for forty-five minutes, though towards 

 the latter part of this period they are less distinct and more limited 

 in extent. The germinal mass has itself during these contractions, 

 which strongly resemble the peristaltic movements of the intestine, 

 undergone changes in form, and has increased in bulk and distinctness. 

 These movements are unaffected by weak galvanic currents. 



During the passage forward of each wave of contraction there is 

 an oscillation of the whole mass of the yelk, so that its germinal 

 pole passes once to the right and once to the left of the micropyle, 

 to which it at first corresponded. The plane of this oscillation may 

 be vertical, horizontal, or inclined, but always cuts the micropyle ; 

 it begins and ceases with the contractions already mentioned, and 

 would seem to be a mechanical result of them. 



For some time before cleavage begins, the only changes of form are 

 the appearance of wave-like elevations and depressions along the 

 under surface of the germinal mass, and its alternate concentration 

 and diffusion. Cleavage begins in about two hours after impreg- 



