DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 



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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. 



(6) 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 parietal 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 ; 



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

 increased during the development of the posterior trunk- 

 region. 



