DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 169 



But, in any event, the accurate study of the earliest embryonic 

 segmentation of the body into a large number of metameres yields 

 this result of the highest importance for the general morphology of 

 the Vertebrate body, that the head not less than the trunk represents 

 a segmented portion of the body and has in no wise been produced 

 from a single primitive segment. 



SUMMARY. 



1. In Vertebrates the middle germ-layers immediately after 

 their origin are differentiated into several fundaments by processes 

 of folding and constricting off. 



2. The process of differentiation in the middle germ-layer exhibits 

 two modifications. 



(a) In Amphioxus the middle germ-layers are, at the time of 



their first appearance, completely separated into primitive 



segments lying one behind the other. 

 It is only later that each primitive segment is divided into a 



dorsal portion (the real primitive segment) and a ventral 



portion. 

 The dorsal portion, or primitive segment proper, furnishes the 



transversely striped musculature of the trunk. 

 The ventral segments form the body-cavity, which is at first 



segmented, but afterwards with the disappearance of the 



partitions becomes a single cavity. 



(b) In all other Vertebrates the fundaments of the middle 



germ-layers are divided first into a dorsal and a ventral 

 region into the primitive-segment plates and the lateral 

 plates. 



The lateral plate remains unsegmented. The body-cavity, which 

 becomes visible in it by separation of the pariotal and 

 the visceral lamellae of the middle layer, is from the 

 beginning on each side of the body a single space. 



The primitive-segment plate alone is divided into successive 

 primitive segments. 



3. The segmentation of the middle germ-layers also extends over 

 the future head-region of the embryo. One therefore distinguishes 



(a) Head-segments, the number of which amounts to nine ; 



(b) Trunk- segments, the number of which is constantly being 



increased during the development of the posterior trunk- 



