No. I.] GERM-LAYERS m CLEPSINE. 125 



The cleavage in Rhynchelmis has been studied with consider- 

 able care by Kowalevsky (No. 10, p. 12), and by Vejdovsky 

 (No. 14, p. 228). Some of the changes which the egg under- 

 goes preparatory to cleavage, as described by Vejdovsky, are 

 of great interest, on account of their manifest identity with cer- 

 tain remarkable polar phenomena which display themselves with 

 great intensity in the (i%^ of Clepsine. I refer to the polar rings 

 of hyaline protoplasm, which concentrate at each pole in the 

 form of a disc. The first two cleavage-planes divide the egg 

 into four macromeres, three of which are nearly equal, and cor- 

 respond to the anterior and the two lateral macromeres of Clep- 

 sine, while the fourth and largest one represents the posterior 

 macromere, and contains most of the hyaline protoplasm of the 

 polar discs, precisely as in the egg of Clepsine. Such a close 

 correspondence in the primary steps of cleavage, resulting in 

 the specialization of one of the four macromeres, affords the 

 strongest possible evidence, short of verification by direct obser- 

 vation, that the subsequent history of this macromere will be 

 essentially the same as that of the posterior macromere in Clep- 

 sine. Unfortunately, the observations on this point are too 

 incomplete to be decisive ; but two important facts may be 

 noted which furnish, at least, a partial verification of the view 

 here taken. First, the median plane of the embryo bears to the 

 first two cleavage-planes relations which are analogous to, if not 

 quite identical with, those described in Clepsine ; and, secondly, 

 a pair of mesoblasts arise from the posterior macromere. 

 Remembering that only two teloblasts were hitherto recognized 

 in Lumbricus, and that a careful study of the germ-bands from 

 surface-preparations has led Wilson to the discovery of neuro- 

 blasts and nephroblasts, it is not venturing much to predict a 

 similar discovery for Rhynchelmis. In the Hirudinea the meso- 

 blasts lie beneath and usually in front of the remaining teloblasts, 

 and their relations to the germ-bands are not easily ascertained 

 without the aid of sections. Most of the remaining teloblasts 

 (neuroblasts and nephroblasts) are very prominent even in the 

 living egg. In the OligocheEta, on the contrary, the mesoblasts lie 

 behind the other teloblasts, and are conspicuous in surface-views ; 



(14.) Vejdovsky, Fr. Die Embryonalentwicklung von Rhynchelmis (Euaxes). 

 Vorluafige Bemerkungen. 



Sitz.-Ber. d. k. b'dhm. Ges. d. fViss. March, 1886. 



