6o6 FOOT AND STROBELL. [Vol. XVI. 



will produce a like morphological effect upon the cytoplasm 

 of the egg .^ We have been unable to differentiate in either 

 spine or middle-piece any special structure that we feel justi- 

 fied in interpreting as a centrosome. 



Further Observations on the Middle-Piece of the Sperm and 

 its Morphological Role in the Male Aster. 



In 1897^ one of us differentiated in color the centrosome 

 of the male aster from the middle-piece of the spermatozoon, 

 this leading to the interpretation that the centrosome of the 

 male aster is of purely cytoplasmic origin — the middle-piece 

 merely producing the cytoplasmic phenomenon known as the 

 aster, but no definite part of the sphere being formed of the 

 middle-piece substance. 



Further investigation has demonstrated that the middle-piece 

 can remain for a definite period intact within the aster, and 

 that the later differentiation in color is probably due to chem- 

 ical change, for it disintegrates before disappearing. 



The photos, of PI. XXXVI show the middle-piece within 

 the aster. Although it is by no means always in the center, 

 we have been able to find no other structure that we feel 

 justified in interpreting as a centrosome. 



The middle-piece finally disintegrates and totally disappears, 

 and there is no evidence that it takes any part in forming the 

 cleavage centrosomes. As stated in the paper above referred 

 to (I), during the formation of the young pronuclei both the egg 

 and sperm centrosomes totally disappear, and there is no evi- 

 dence that either takes part in forming the cleavage centrosomes, 

 this egg supporting the theory that the cleavage centrosomes 

 arise de novo in the cytoplasm. 



In photo. 12 we see a portion of the head of the spermato- 

 zoon, its middle-piece within the aster, and a part of the tail 

 (the missing part of the head is in the next section). The 

 head at this stage shows constrictions at definite intervals 

 (these are lost in some of the reproductions of photo. 12, but 



^ Foot, " The Origin of the Cleavage Centrosomes," /(??<r«. of Morph., vol. xii. 

 No. 3, 1897. 



