404 GEORGE W. TANNREUTHER 



posed of cellular processes (text fig. k), which, later, as develop- 

 ment progresses, lose all traces of cell boundaries, with a few 

 nuclei persisting (text figs, i to m). The method of attachment 

 in one of the completely formed muscles is represented in text 

 figure 1. It is attached anteriorly to the corona and at its opposite 

 end it is anchored to the ectoderm. Some of these muscle fibers 

 show a distinct cross or longitudinal striation with few nuclei. 

 In the mature embryo (text fig. n), many of the muscle fibers are 

 drawn out and form a delicate network, which extends through 

 the different regions of the body cavity. Some of these fibers 

 are mere lines and are hard to distinguish. Many of the apparent 

 migratory cells within the fluid of the body cavity are directly 

 connected with very delicate processes (text figs, m and n), 

 while others are mere floating corpuscles, which are highly 

 vacuolated. The various structures within the body, as well as 

 the trochal disc, are kept in constant motion by the activities 

 of the various muscle fibers. No attempt was made to represent 

 the position of the different muscles in the various figures drawn. 



3. Entoderm 



During the early part of the seventh cleavage, the blastopore 

 becomes completely closed (figs. 59 and 60). The cells desig- 

 nated as the entoderm include the large cell E and the two small 

 cells d* and d% which are formed from E during the sixth and 

 seventh cleavages of the embryo. Figures 58 to 60 show the 

 first stages in the cleavage of E, which corresponds to the eighth 

 cleavage. This cleavage is unequal and separates a smaller cell, 

 E^, from a larger cell, E^, posteriorly. Figure 60 is a dorsal 

 view of the embryo and shows the condition of the ectoderm 

 and entoderm at the beginning of the eighth cleavage. The 

 ectodermal cells during the ninth cleavage divide very rapidly 

 and are difficult to follow. Immediately after the first cleavage 

 of E, the cell E^ divides equally and transversely, or at right 

 angles to its first cleavage. Figures 61 to 64 show the different 

 stages in the process of division. The embryo, as a whole, is 

 very plastic and becomes more spherical during the cleavage of 



