EMBRYOLOGY OF CLEPSINE. 263 



two processes are identical, and, if in one case the object is 

 reproduction, how can we say that in the other it is simply to 

 get rid of a part of the nucleus ? According to the view I 

 have taken, the production of polar globules, or something very 

 analogous, in thf; formation of spermatozoa, as Strasburger 

 (TirTTgWo-iTj-nr, TTj-fnr) lias shown, is nothing surprising. 

 Such eifete formations are the results of abortive efforts to 

 reproduce in the original way. I should be compelled perhaps 

 to abandon this theory if polar globules should be found in 

 ^gs that develope, either exceptionally (moths) or regularly 

 (case of drones among bees), without impregnation. In the 

 case of Neritina (Biiischli o-yy-T) ^li^ unfecundated eggs are said 

 to produce polar globules, and then, after performing a number 

 of irregular cleavages, break up and serve as " food-material " 

 for the single developing ovum. There are two unsettled points 

 here. According to Professor Lankester (t?-^), only one egg is 

 subject to cleavage ; and Biitschli admits an uncertainty in 

 regard to whether both sorts of eggs are impregnated or not. 

 Should it turn out, however, that in this case unfecundated eggs 

 both produce polar globules and cleave, it would then be possible 

 to explain the anomaly on the supposition that an event palin- 

 genetically introduced tends to repeat itself even after the^ 

 cenogenetic cancellation of the factor by which it was intro- 

 duced. This is illustrated by the appearance of cleavage in the 

 ^^^^ of birds and Echinoderms, even when fecundation is 

 omitted. We should then have to assume that originally all 

 the eggs of a capsule developed embryos. 



IV. Cleavage. 



The importance of accurate and detailed study of the cleavage 

 process is well illustrated by the brilliant results attained by 

 Kowalevsky (Euaxes), van Beneden (Rabbit), and Eabl (Unio). 

 In the fecundated egg slumbers potentially the future embryo. j 



While we cannot say that the embryo is predelineated, we caa^ ^ 



say that it is predetermined. The " Histogenetic sundering " 

 of embryonic elements begins with the cleavage, and every step 

 in the process bears a definite and invariable relation to antece- (" ' 



dent and subsequent steps ; or, as Bergmann and Leuckart have \ / 



expressed it, " Jeder einzelne Entwicklungs moment is die noth- 

 wendige Folge des vorausgegangenen und die Bedingung des 

 folgenden" (-f^) It is, therefore, not surprising to find certain 

 important histological difl'erentiations and fundamental structural 

 relations anticipated in the early phases of cleavage, and fore- 

 shadowed even before the cleavage begin^. 



The egg is, in a certain sense, a quarry out of which, without 

 waste, a complicated structure is to be built up; but more than 



VOL. XVIII. NEW SER. S 



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