284 



Another problem which presents itself is to determine at what 

 stage the balancer-forming region of the embryo becomes definitely 

 localized. At what stage does the tissue become so differentiated 

 that it can form no other structure except the balancer? The trans- 

 plantation method may also throw some light on this problem. 



The technique of the operations was similar to that employed in 

 previous experiments i). Embryos of Diemyctylus viridescens (3 mm 

 to 10 mm long) were used. 



A brief account of the structure and the development of the 

 balancer is given here to facilitate the explanation of the experiments ^). 

 The balancer anläge is first noticeable in embryos 4 mm to 5 mm 

 long as a slight elevation about in the mid-lateral line, in front of 

 the area from which the gills develop. It soon becomes a short pro- 

 jection (6 mm long, Fig. 1). Later, 7,5 mm (Fig. 2) it is a process 



Fig. 1. 



Fig. 2. 



Fig. 1. Anterior part of embryo as seen from the left side. Length 6 mm. 

 b balancer, e eye. g anläge of external gills, n nasal pit. X 27. 



Fig. 2. Anterior part of embryo as seen from the left side. Length 7,5 mm. 

 b balancer, e eye. g external gill. 7i nasal pit. X ^7. 



of nearly uniform diameter directed caudally and outwards. At a 

 length of 9 mm to 10 mm the embryos break out of their gelatinous 

 envelope and the balancers are made use of at once for holding to 

 the sides and bottom of the dish and for supporting the body. 



The embryos were brought into the Laboratory from the cooling 



1) Arch. f. Entw.-Mech., Bd. 23, 1907, Heft 3, p. 457. 



2) I was not able to find a discussion of the histology or the 

 histogenesis of the organs in the literature, though it is easily possible 

 that such exists. 



