414 GEORGE W. TANNREUTHER 



vacuole and the tubules. At first the urinary bladder shows a 

 distinctly nucleated wall (text fig. s), but later, as enlargement 

 continues, the cells are no longer distinguishable and the wall 

 becomes more membranous, as in the adult. Text figure t, an 

 optical section of a fourteen-hour embryo from the left side, 

 shows the relation of the different parts of the excretory system. 

 The movement of a finely granular substance within the lumen 

 of the tubules is visible under high magnificati(3n. This flow of 

 substance is due to the action of the cilia within the flame cells 

 and the activities of the delicate fibers in the tubule walls. The 

 constant rhythmical contraction and expansion of the urinary 

 bladder contribute to the process of elimination of the excretory 

 products. In the formation of the excretory system on the 

 ventral side, its early rudiments do not lie free in the body 

 cavity but are directly connected with the ectoderm at different 

 points by means of cells, which originated from the ectoderm 

 (text figs, q and r). These cell connectives later develop into 

 the muscles, which connect the forming tubules with the body 

 wall. These tubules at first are on the ventral side of the body 

 cavity, but later, as their associated muscle fibers develop, the 

 excretory tubules may occupy almost any part of the body 

 cavity, depending upon the contraction and expansion of their 

 controlling muscles. 



NERVOUS SYSTEM 



The brain is formed by a solid ingrowth or proliferation of 

 cells from the ectoderm at the anterior end of the embryo, dorsal 

 to the mouth (figs. 96 and 98). A small portion of the brain on 

 the dorsal side gives rise to the rudimentary eye, which appears 

 as a red pigmented body. The different intensities in the pigment 

 formation of the eye can be recognized in the developing embryo. 

 The origin of the nerve fibers can be distinguished in section (fig. 

 103). This figure shows the origin of the lateral and anterior 

 nerves, which innervate the trochal disc. Each of the lateral 

 trunks gives off a branch which runs nearly the entire length of 

 the animal. These nerves are hard to demonstrate except in 

 the close proximity to their origin. 



