332 JOHN BEAED, 



Description of the apparatus iii a series of embryos 



of -B. batis, 



1) Embryos of 5 — 11 mm. 



An embryo of R. hatis of 5 mm length (No. 45 of the collection) 

 shows the following. There are 45 trunk somites behind the audi- 

 tory region. The primary optic vesicles reach the skin, but there is 

 no trace of the lens. The auditory organ is represented by a flat 

 patch of epiblast. The stomatodeal involution is still intact. One 

 gill-pouch is present, and this has not yet broken through the epi- 

 blast ^). The pronephros is in course of formation, and as yet no 

 part of the segmental duct has arisen. The ganglionic foundations 

 or "Anlagen" (spinal and cranial) are still quite within the lips of 

 the central apparatus. Of the transient cells to be afterwards de- 

 scribed none have taken on ganglionic characters and there are no 

 nerve processes. In the words of His, the entire nervous system is 

 as yet a system without ganglion-cells and without nerves. 



A slightly older embryo (R. batis, No. 134) measured 5,25 mm, 

 and possessed 54 — 55 trunk somites. The general characters of its 

 eye and ear are those of a succeeding embryo (No. 135). A third 

 gill- pouch is in course of formation as an outgrowth from the gut. 

 The pronephros is as far advanced as in No. 135, but the segmental 

 duct hardly extends so far caudad. In this specimen the ganglionic 

 foundations are only just commencing their exodus from the lips of 

 the cord, and this in the region of the pronephros. Here and there 

 a cell with commencing ganglionic characters may be noted, but 

 there are as yet no nerve-processes. 



Fig. 5, pi. 22, is taken from the pronephric region of this embryo. 

 It shows the dorsal region of the cord, and the ganglionic founda- 

 tions are seen to be in course of migration outwards. Among these 

 migrating cells there are three (glc), which have made progress in 

 the direction of becoming ganglionic. They stain less deeply than 

 do the other cells, the nucleus is large and rounded, and it contains 

 two or more refractile nucleoli. 



1) In the formation of the gill-slits there appears to be no actual 

 fusion with the epiblast. The hypoblastic pouch is pushed against the 

 latter, which thins out and finally ruptures at the same time that the 

 outer wall of the pouch itself parts. 



