402 GEORGE W. TANNREUTHER 



the fifth cleavage. Each cell divides unequally. The anterior 

 cell divides vertically, while the other two divide transversely 

 and parallel to the long axis of the body (fig. 48). For further 

 account of the cleavage process in the formation of the early 

 ectoderm, see description given for the later cleavage stages. 

 Figures 55 to 57 represent the earlier stages in which the cleavage 

 cells become arranged into a definite epithelial layer. The cavity 

 shown in these figures is in reality the early body cavity, since 

 the epithelium later becomes the definitive body wall. A later 

 condition of the ectoderm is shown in figures 67 and 68. From 

 this point onward the division of the epithelial cells is very 

 irregular and their products become differentiated into the 

 definitive ectoderm and its various derivatives. At first the 

 definitive ectoderm is represented by an epithelial layer com- 

 posed of large nucleated cells, and is in immediate contact with 

 the organs forming within (figs. 77 to 90). But as the different 

 organs reach their complete development, the body cavity 

 becomes more and more marked and the cells of the ectoderm 

 are drawn out into a very thin epithelial layer, with the boundary 

 of the cells no longer visible (figs. 96 to 104). The cuticle is a 

 very thin layer formed by a secretion from the ectoderm shortly 

 before birth. The cuticle is usually free from markings and has 

 a smooth surface. Figure 104 represents the condition of these 

 various structures at the time of birth. 



2. Mesoderm 



In the late gastrula stage (figs. 63 to 70), the outer epithelial 

 layer of the embryo at different regions begins the prohferation 

 of cells on the inner surface. These cells contribute directly to 

 the formation of the mesoderm, which later becomes differen- 

 tiated into the muscular system. It is impossible to distinguish 

 in the embryonic ectoderm the cells which directly give rise to 

 the mesoderm. Apparently all parts of the ectoderm possess 

 the same potentiality in the process. These cells, in their for- 

 mation, remain connected with their point of origin. Later these 

 proUferated cells show an apparent connection or fusion with the 



