704 JOHN BEARD, 



the nature and mode of Metazoan development, entered upon by the 

 writer some thirteen years ago. But, as every experienced embryo- 

 logist knows, the starting point of a research often becomes the closing 

 chapter of the narrative, while what should have been the com- 

 mencement only reveals itself at the finish, if at all. 



As demonstrated in the following pages, the prelude to every 

 developmental history, usually played long before the actual story 

 begins to unfold itself, is the determination of sex. The account to 

 be here presented of the latter is of so simple and obvious a character, 

 that it is difficult to understand why it was not found out long ago. 

 The writer has no desire to give undue prominence to his own finds: 

 indeed, he would suggest, that without them evidences of the actual 

 existence of fourfold gametes, two sorts of eggs and two kinds of 

 spermatozoa, sufficient to permit of the solution of the problem, have 

 long been known. At any rate, their occurrence should have served to 

 indicate the way. 



Two sorts of eggs, sexually differentiated and of different sizes, 

 have been known for many years. They were recorded for one species 

 of Rotifer by J. Dalrymple in 1849, for other species by F. Leydio 

 (1854) and, especially, by F. Cohn (1856), by Balbiani for Phylloxera 

 coccinea (1873), and for Dinopkilus gyrociliatus by E. Korschelt 

 (1882). Cohn's services in this direction were great, for he realised 

 more clearly than his predecessors the significance of his finds ^). 



Two kinds of spermatozoa were originally reported by von Siebold 

 in 1836. For long after the discovery their occurrence found no ad- 

 mission into the Acta of the science; and, without examining the 

 actual facts in the animal concerned, Kölliker disposed of them by 

 denying the correctness of Siebold's observation ! Later on Leydig 

 was able to confirm the statements of the latter zoologist, but his 

 work and the subsequent investigations of Duval, M. von Brunn, 

 Auerbach, and F. Meves have hardly yet succeeded in finding a certain 

 place in embryology for the worm-like spermatozoon of Paludina. The 

 ordinary or hairlike and functional sperm is admitted: its remarkable 

 brother is still either entirely ignored, or, if mentioned, then only as 

 a curiosity. 



And why? Because, forsooth, apart from the little understood 

 hermaphrodites, there are but two kinds of individuals in the Metazoa, 

 the males and the females. And, as the gametes, the eggs and sperms. 



1) in: Z. wiss. Zool, V. 7, 1856, p. 450. 



