THE ORGANS OF THE MIDDLE GERM-LAYER 343 



increase considerably in size, project far into the cavity of the 

 primitive segment, and assume the form of plates ; these lie parallel 

 to one another and to the longitudinal axis of the body ; and one 

 margin, which I shall designate as the base, is placed perpendicularly 

 upon the surface of the chorda. Very early (in the stage with ten 

 primitive segments) the cell-plates begin at their bases to be differ- 

 entiated into transversely striped muscle-fibrillfe, with which the 

 embryos are already able to execute feeble contractions. By the 

 continual addition of new fibrillse to those which are formed at the 

 surface of the chorda, and by an extension of the differentiation to 

 both the surfaces of the cell-plates 

 which are in contact with each other, 

 there arise the transversely striped 

 musde-layera (Muskelblatter) which 

 are characteristic of the musculature 

 of Amphioxus. These are attached 

 to the chorda on the right and left 

 like the leaves of a book. The more 

 the fibrillse increase in number, the 

 more the protoplasm of the forma- Fig . isa. -Cross section through the 

 tive cells between them diminishes in middle of ^ bod y of an Am - 



phioxus embryo with 11 primitive 



amount and the more IS the nucleus segments, after HATSCHEK. 



With a remnant of protoplasm forced M*. Outer inner germ-layer ; rf, 



. /*", parietal, visceral lamella of 



toward that edge OI the Cell which the middle germ-layer ; us, primi- 



faces the cavity of the primitive ti * eg ? ent ,, : * eura ! tn ^ : 



ch, chorda ; th, body-cavity ; dh, 



Segment. intestinal cavity. 



The remaining cells of the primitive 



segment are converted into a low pavement-epithelium, which neither 

 now nor later takes part in the formation of muscles. (Cutis-layer 

 of HATSCHEK.) 



Having arisen in the vicinity of the chorda, the muscle-layer in 

 older animals spreads out both dorsal ly and ventrally, and thus 

 furnishes the total voluminous musculature of the trunk, which, like 

 the cellular primitive segments from which it is derived, is separated 

 into successive portions (the myomeres). 



In general the Cyclostomes (fig. 190) agree in the development of 

 their muscles with Amphioxus. Here, as there, one must distinguish 

 between an inner muscle-forming epithelial layer (mf), which bounds 

 the chorda (Ch) and the neural tube (jV), and an outer indifferent 

 epithelial layer (ae), which occupies the side toward the epidermis. 

 The latter (ae) consists of low flat cells, the former of very broad and. 



