364 EDWIN G. CONKLIN 



nothing new to add to those conclusions I need only say that the 

 flowing movements which I there described may be interpreted 

 as the result of the contractility of the spongioplasm, — which is 

 indeed the original explanation of these movements (Van Bene- 

 den '87, Boveri '87). 



4. The orientations of development 



According to the view here expressed the localizations of 

 spindles and cleavage planes, of nuclei and centrospheres, of 

 cytoplasm and yolk, and indeed the orientation of all develop- 

 mental processes is associated with the structure and activities 

 of the spongioplasm. In eggs generally cytoplasm becomes con- 

 centrated at the animal pole during early stages of development 

 and coincidently yolk is forced away from that pole, probably 

 by contraction of the spongioplasm to the animal pole; nuclei 

 and centrospheres are bound together and are held in a definite 

 relation to the animal pole by strands of spongioplasm; mitotic 

 figures are oriented by means of the framework of spongioplasm 

 and the planes of cleavage are thereby determined. 



In all cases the position and direction of the division planes is 

 controlled by the position and direction of the spindle in the later 

 stages of mitosis, the division plane always passing through the 

 equator of the spindle and at right angles to its axis. In nor- 

 mal eggs of Crepidula the first maturation spindle forms in the 

 position previously occupied by the germinal vesicle — a little 

 removed from the surface of the egg. This spindle reaches its 

 maximum length in the metaphase at which time it is about as 

 long as the radius of the egg. In the anaphase the peripheral 

 pole of the spindle comes into close contact with the peripheral 

 layer of protoplasm and at the same time the aster at this pole 

 grows smaller and smaller and is at last completely absorbed 

 into the periphei'al layer while coincidently the spindle grows 

 shorter so that when the division wall is formed through the 

 equator of the spindle it cuts off a very small polar body from a 

 relatively enormous egg. On the other hand where both asters 

 are attached to the peripheral layer as in certain cleavages, the 



