770 



The American Naturalist. 



[September, 



could it produce any wrinkles at all in the layer itself? Would it not 

 rather tend to flatten the superior pole and stretch the viscous layer 

 tightly over the underlying protoplasm in a manner similar to the 

 action of the muscles of the diaphragm during respiration ? 



As to the difference in contractility between the outer and inner 

 protoplasm, it is evident that if this is to smooth out the wrinkles, the 

 difference must be in favor of the outside layer, and also the contraction 

 must be at right angles to the length of the wrinkles. That means in the 

 present instance that it must be parallel to the first furrow. Neither 

 of these conditions seem possible, and we thus find both explanations 

 inadequate when confronted by the facts in the case. They both fail 

 to account for the mass of pigment under the superior pole also. 



Bambeke states that" the entrance of cortical pigmented masses into 

 the interior of the egg . . . can only be explained by admitting the ex- 

 istence of contractions in the ovular protoplasm during segmentation." 



According to the interpretation of the present day, cell division is 

 brought about by means of some force or forces acting along the length 

 of the segmentation spindle. In the present instance this spindle was 

 formed between the two nuclei represented in fig. 39, which lie some 

 distance below the surface of the egg. 



If we interpret Bambeke's "contraction of the ovular protoplasm " 

 to be identical with this working force of the segmentation spindle, it 

 will explain the presence of the lunate mass of pigment directly over 

 the spindle, and will help us to understand the presence of a pigmented 

 layer on either side of the segmentation furrows. But it does not ex- 

 plain in any way the formation of the wrinkles. 



Reichert's wonderfully inconsistent explanation of the origin of the 

 wrinkles is quoted, and sufficiently commented upon by Schultze, in the 

 paper already referred to. 



In view of the fact, therefore, that we have no explanation which 

 can stand the test of our present knowledge of cell division, we venture 

 to offer the following : 



We agree with Schultze that the external pigmented layer is neces- 

 sarily somewhat denser that the internal protoplasm. 



Modern research indicates that this layer is drawn inward in some 

 way, by the forces working along the segmentation spindle, to form the 

 furrow which lies over the equator of the spindle. The bottom of the 

 furrow, thus formed by an infolding of the surface layer, describes an 

 arc which becomes shorter and shorter as the furrow deepens. 



This shortening of the arc must result in one of two things. The 

 bottom of the furrow may remain of the same length as at first, and 



