THE ORGANS 01 KB! Ml DIM. I. OBftM-LAYBR. 351 



/;. rit<> Segment* 



Important woi k> on the de\.-lopiii.-nt of tlio head have appeared 

 in Late years by GOUTS, B.M.FOIK. M \K-II M.I.. \\Uiii:. I-'KOKI 

 and others. They have led to the impi i.-mt conclusion tin' 

 head is made up of a large number of segments, in the same manner 

 as the trunk. These conditions are HUM evident in the Selachians. 



When in these animals the middle germ layer- have grown into 

 the fundament of the head, they here, as in the trunk, early sepa rat 

 from each other, and thus embrace on either >ide a nan-, 

 like space, the head cavity. This is continuou> pnMeriorly with the 

 general l>ody-cavity. It follows from this that 

 the two primitive body-sacs (ccelom-sacs) possess 

 <> <jreater extent in the embryo than they do sub- 

 *nfiit>nttii, since they reach into the most anf> ru>r 

 part of the embryonic fundament, the head. aa- 



In the further course of development the walls 

 of the head-cavity are differentiated, in the same Fier. i96.-Cross section 

 manner as the walls of the body-cavity, into a through the next to 



J J1 the last visceral arch 



ventral portion and a dorsal portion, the latter of an embryo of Pris- 



producing primitive segments. Then there arises, 



however, an important difference between head visceral 



and trunk ; in the trunk only the dorsal portion 



is segmented, but in the head both ventral and visceral arch ;oa, blood- 



, ,. , , , vowel of the visceral 



dorsal portions are segmented, each in a manner arch ( aortic arch ) 

 peculiar to itself. 



The ventral part of the head-cavity is divided, in consequence 

 of the development of the \isceral clefts, into separate segments 

 (hranchiomeres AHLBORN), the first of which is situated in front of 

 the first cleft, each of the remaining ones between two clefts. Each 

 segment (tig. 196) consists of a wall composed of cylindrical cells and 

 encloses a narrow cavity. With its enveloping connective tissue it 

 constitutes the vi-eeral aivh B, which are separated from one another 

 by the visceral clefts; for this reason the fissures arising from the 

 head-cavity have been designated by \\ "I.HII: as vi-c.-ral-arch cavities. 

 The latter communicate for a time under the gill-pouches with the 

 pericardial chamber surrounding the heart. But then they begin to 

 be closed; their walls come into contact ; and out of the cylindrical 

 epithelial cells are developed the tran>\cr>-ly striped muscle fibres 

 which produce the muscles oftliejaws an>l <////*. 



Consequently there results for the head-region of Ver this 



