GENERAL SKETCH 143 



a very convincing manner not only that the amphiaster is a product 

 of the sperm-aster, but also that the latter is developed about the 

 middle-piece as a centre. The same result was indicated by Foot's 

 observations on the earthworm ('94), and it was soon afterwards 

 conclusively demonstrated in echinoderms through the independent 

 and nearly simultaneous researches of myself on the egg of Toxo- 

 pnenstes, of Mathews on Arbacia, and of Boveri on Echinus. Nearly 

 at the same time a careful study was made by Mead ('95) of the 

 annelid Chceopterus, and of the starfish Asterias by Mathews, both 

 observers independently showing .that the polar spindle contains dis- 

 tinct centrosomes, which, however, degenerate after the formation of 

 the polar bodies, their place being taken by the sperm-centrosome, 

 which divides to form an amphiaster before union of the nuclei, as 

 in Rhynchelmis. Exactly the same result has since been reached by 

 Hill ('95) in Sphczrechimis and the tunicate Phallusia, and by Kos- 

 tanecki and Wierzejski ('96) in Physa (Fig. 64) ; and in all of these 

 the centrosome is likewise shown to arise from the middle-piece. 

 The origin of the centrosome from the spermatozoon alone has also 

 been shown by Ruckert ('95, 2) in Cyclops (Fig. 72), and is indicated 

 by Sobotta's work ('95) on the fertilization of the mouse (Fig. 67). 



Such an array of evidence, derived from the study of so many 

 diverse groups, places Boveri's conception of fertilization (p. 141) on 

 a very strong foundation, and justifies the conclusion that the origin 

 of the first cleavage-centrosomes from the spermatozoon alone is a 

 phenomenon of very wide, if not of universal, occurrence. The 

 descendants of these centrosomes may be traced continuously into 

 later cleavage-stages, arid there can be little doubt that they are the 

 progenitors of all the centrosomes of the adult body. Boveri and 

 Van Beneden, followed by a number of later observers, 1 have followed 

 the daughter-centrosomes through every stage of the first cleavage 

 into the blastomeres of the two-cell stage, where they persist and give 

 rise to the centrosomes of the four-cell stage, and so on in later stages. 

 This is beautifully shown in the egg of Thalassema (Fig. 73), which 

 has been carefully followed out in my laboratory by Mr. B. B. Griffin. 

 The centrosome is here a minute granule at the focus of the sperm- 

 aster, which divides to form an amphiaster soon after the entrance of 

 the spermatozoon. During the early anaphase of the first cleavage 

 each centrosome divides into two, passes to the outer periphery of the 

 centrosphere, and there forms a minute amphiaster for the second 

 cleavage before the first cleavage takes place (Fig. 73) ! The minute 

 centrosomes of the second cleavage are therefore the direct descendants 

 of the sperm-centrosome ; and there is good reason to believe that the 



1 See Mead on Chtropterus, '95, and Kostanecki and Wierzejski on Physa, '96. 



