DEVELOP.'.!!'. NT (K illl. TIM -T3. 1GQ 



F.iit, in anv event, ;! mbryonic 



'ntation of the body into a large iiuiiil>er of metameres yield-; 

 this result of the highest importance forth" gem-nil morphology of 

 the Vertebrate body, that tl,,> ln-,1,1 not fat '/>>, //,< t r mi k represent* 

 portion of the bod// tm</ Imx in 'no VIM bv.'u produced 

 primitiw 



SUM .MA UN. 



1. In Vertebrates the middle germ-layors immediately after 

 their origin art- differentiated into several fundaments by prc<-> 

 of folding and constricting off. 



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

 two modifications. 



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

 their first appearance, comjiii'k'lii 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 lir.-t 

 segmented, but afterwards with the disappearance of the 

 partitions becomes a single cavity. 



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

 germ-lay ITS are divided first into a dorsal and a ventral 

 region into the primitive-segment plates and the lat- 

 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 sp: 



The primitive-segment plate alone is divided into successive 



primitive segmoi 



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

 the future head-region of the embryo. One therefore di.-tingui>he> 



(a) Head-segment*, 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- 



