142 whitman: [Vol.1. 



and nephroblasts, but also one of the four primary micromeres. 

 The teloblasts stand thus in the direct Hne of descent with the 

 acroblasts, and are at first in close contact with them. The full 

 significance of the teloblasts and their original relations can only 

 be made clear by comparison with the larval forms of other 

 annelids. Farther on I shall indicate briefly some points in this 

 comparison. 



Germ-bands of the Trunk. — Each germ-band consists of 

 three distinct layers: (i) A thin epidermal layer, (2) a 

 neuro-nephric layer, and (3) a mesoblastic layer. The character 

 and relative positions of these layers may be seen in Figs. 2, 6, 

 and 7, PI. IV. The epidermal layer (r/) consists of flattened 

 cells, more deeply stained with osmic acid than the underlying 

 strata. The neuro-nephric layer is represented by four longi- 

 tudinal rows of cubical or oval cells {11c, np/i, and m'), as is best 

 seen in surface views (Fig. 8, PI. V.). The mesoblastic layer 

 {in) consists of large, rounded, or polygonal cells, two or more 

 deep, filling the space between the neuro-nephric layer and 

 the yolk. 



Origin of the Ventral Nerve-Chain. — As my observations 

 on the origin of the nerve-chain contradict those of Kowalevsky 

 and Nusbaum, and as they do not confirm the anticipations of 

 such clear-sighted embryologists as Balfour, I can hardly do 

 justice to the subject without dealing briefly with its historical 

 side. 



(«) Historical and Critical. — Filippi (No. 19, p. 23), the 

 earliest writer on the embryology of Clepsine, tells us that it 

 is impossible to trace the origin of individual organs, owing to 

 the small size of the embryos. 



Grube (No. 2, p. 35) derived the nerve-cord from the germ- 

 bands (" BauchwUlsten "), the defective technique of the times 

 not enabling him to reach more definite results. 



In the posthumous work of Rathke, edited and revised by 

 Leuckart, the nerve-chain is said to be formed from the median 

 part of the germ-bands (" Bauchplatten "). This result, obtained 

 long before the introduction of the microtome, comes much 

 nearer the truth than the statements of Kowalevsky or Nus- 

 baum. Without the aid of sections, the inner stratum of the 



(19.) P'lLiPPi, F. DE. Sopra I'Anatomia e lo Sviluppo delle Clepsine. Pavia, 

 1839. 



