342 EMBRYOLOGY. 



of difficulty, because since the time of REMAK there ha been an 

 endeavor to bring the middle germ -layer as a non-epithelial structure 

 into contrast with the other germ-layers. Attempts were also made 

 to explain this supposed contradiction by assuming that the glandular 

 organs in question were derived, sometimes in one way, sometimes in 

 another, from the outer germ-layer. With the acceptance of the 

 ccelom-theory, however, the theoretical objections to the production 

 of glands by the middle germ-layer have ceased to have any 

 foundation. 



Out of the middle germ-layer, or, otherwise expressed, out of the 

 epithelial wall of the embryonic body-sacs, are developed aside from 

 the mesenchyme, concerning the source of which an extended account 

 was given in the ninth chapter three very different products : first 

 the whole voluntary musculature, secondly the urinary and sexual 

 organs, thirdly the epithelial or endothelial linings of the large 

 serous cavities of the body. 



I. The Development of the Voluntary Musculature. 



The total, transversely striped, voluntary musculature, aside from 

 a part of the muscles of the head, arises from those parts of the 

 middle germ-layer which have been differentiated as primitive 

 segments, and with their appearance have effected the first primitive 

 and most important segmentation of the vertebrate body. As has 

 been previously stated, the segmentation affects the head as well as 

 the trunk, so that trunk-segments and head-segments must be dis- 

 tinguished. Since the latter are in many points distinguished in 

 their origin and metamorphosis from the former, a separate descrip- 

 tion of the two is fitting. I begin with the history of the metamor- 

 phosis of the primitive segments of the trunk, and treat of the same 

 first in Amphioxus and the Cyclostomes, which furnish the simplest 

 and most easily interpreted conditions, and then in the Amphibia, 

 and finally in the higher Vertebrates. 



A. Primitive Segments of the Trunk. 



In Amphioxus the primitive segments (fig. 103 u$h) are sacs, which 

 are provided with a large cavity, and the walls of which are composed 

 of a single layer of epithelial cells. The latter are further developed 

 in two ways, for an accurate knowledge of which we are indebted to 

 HATSCHEK. Only the cells (fig. 189) which abut upon the chorda (ch) 

 and the neural tube (n) are destined to form muscle-fibres ; they 



