No. 3] THE PROTOZOA AND METAZOA. 735 



separate small cell around itself. It then passed into the 

 resting stage, from which it emerged to form a mitotic-figure 

 in which all of the elements, save centrosomes, were present. 

 In the second place, a number of observers have maintained 

 that, for each mitosis, the centrosome arises de novo, as in the 

 well-known theories of Burger ('92), Watasd ('93), Reinke ('94), 

 and others. The results which Hertwig ('95) and Morgan ('96) 

 obtained by treating the cell with dilute poisons, etc., are evi- 

 dence in the same direction. Foot ('97) also holds that the 

 centrosome in the first cleavage-spindle of Allolobophorafoeiida 

 is the physiological effect of the spermatozoon upon the cyto- 

 plasm of the egg, and not a permanent morphological element 

 brought in by the spermatozoon. To this list must be added 

 a great number of observers who have followed the history of 

 the centrosome up to a certain point in cell changes, after 

 which they found no further trace of it. Finally, many 

 observers have followed the centrosome from one generation 

 to another and maintain it to be a permanent cell organ in 

 accordance with the original view of Boveri ('87) and Van 

 Beneden ('87). Brauer ('93) and the majority of spermatologists 

 have traced the centrosome from generation to generation, in 

 some cases up to a certain definite structure in the mature 

 spermatozoon (Moore ('93), Meves ('96), Hermann ('92), Cal- 

 kins ('95)), and many observers have followed the centrosome 

 in the same way in the cleavage of the egg (Boveri ('87), Van 

 Beneden ('87), Henneguy ('91), Mead ('95),i Griffin ('96)2). 



From this review of the subject it is plain that the question 

 of permanency of the centrosome has no immediate prospect 

 of settlement. Repeated examinations of my sections, the 

 evidence of which I have given above, I believe, justifies the 

 conclusion, in a provisional sense at least, that the centrosome 

 of Noctiluca is a permanent element ; that it exists during the 



1 Mead later ('97) comes to quite an opposite view. He says : " The fore- 

 going observations convince me that the asters and centrosomes in the Chaetop- 

 terus ovum arise by modification of the cytoplasmic reticulum " (p. 394). 



'^ Wilson ('97) has traced the sperm-centrosome through the first cleavage and 

 into the 2-celled stage in Arbacia. Contrary to the view of Doflein ('97), he finds 

 that the centrosome is not represented by the entire middlepiece of the spermato- 

 zoon, but that it is contained within the middlepiece which is thrown off as a 

 shell after the spermatozoon enters the egg. 



