Developmental history of primary segments of the vertebrate head. 423 



appear, as represented in Fig. 45. This figure shows a horizontal 

 section through the left half of the neural segments, 6—9 of a chick 

 embryo 42 hours old. At this age all the characteristics of a typical 

 "neuromere" as described by Orr are present. 



1) "Each neuromere is separated from its neighbor by an external 

 dorso-ventral constriction and opposite this an internal sharp dorso- 

 ventral ridge, — so that each neuromere (i. e. one lateral half of each) 

 appears as a small arc of a circle. 



2) The elongated cells are placed radially to the inner curved 

 surface of the neuromere. 



3) The nuclei are generally nearer the outer surface and approach 

 the inner surface only towards the apex of the ridge. 



4) On the line between the apex of the internal ridge and the 

 pit of the external depression , the cells of adjoining neuromeres are 

 crowded together, though the cells of one neuromere do not extend 

 into another neuromere. 



This definition of adjacent neuromeres presents in some sections 

 the appearance of a septum extending from the pit of the external 

 depression to the summit of the internal ridge" (Orr, '87, p. 335). 



In addition to the above characteristics I find a transverse groove 

 (g' and f, Fig. 45), at the apex of many of the internal ridges or 

 crests. The internal transverse grooves described in embryos with 

 open neural grooves have thus become elevated, in the medulla of the 

 chick, and occupy a position at the apices of the internal transverse 

 ridges. A discussion of this observation appears in another portion 

 of this contribution. 



Fig. 46 represents a horizontal section of the medulla of a trout 

 embryo 27 days old, a stage when the neural segments are highly 

 developed. It will be observed that the dividing lines between seg- 

 ments are represented by external and internal transverse grooves 

 that lie in nearly the same transverse plane. The cells are elongated 

 and arranged radially to the internal surface. The nuclei are large 

 and well supplied with chromatin. The septa that separate adjacent 

 segments are usually very distinct and, as Orr has stated, may be 

 nothing more than the walls of adjacent cells that are placed in 

 straight lines. The histological condition of the neural segments of 

 the chick embryo (Fig. 45) is identical with the description given above. 



In younger embryonic stages of the chick and the trout, the 

 histology is very simple. The radial arrangement of cells is absent. 

 The nuclei do not recede intrasegmentally from the inner surface of 



Zool. Jahrb. xm. Abth. f. Morph. 28 



