268 GEO. W. TANNREUTHER. 



from side view in the living material. The third cleavage 

 furrow which passes entirely through the egg instead of becom- 

 ing obliterated within, as in the first and second cleavages, be- 

 comes the cleavage cavity. PI. IX., Fig. 13 represents a section 

 passing through the poles of the egg shortly after the third 

 cleavage is complete. The cleavage cavity is very distinct and 

 shows the nuclei near the inner ends of the cells. The egg now 

 becomes more spherical and the inner ends of the cells become 

 rounded off. The blastomeres of the vegetative pole are larger 

 than those of the animal pole. The fourth and fifth cleavages 

 are parallel to the third, but there is some irregularity in their 

 time of formation. Both cleavages may begin at the same time, 

 but in most instances observed the cells above the equator divided 

 first. They differ from the first, second and third cleavages in 

 that the cleavage does not start at one side and gradually pass to 

 the opposite, but instead begins at different points on the surface 

 at the same time. The cleavage furrows are very indistinct. PI. 

 X., Fig. 14 represents the fourth and fifth cleavages completed, 

 as they appear in a plane passing through the poles. The cleav- 

 age cavity is very irregular and increases considerably in size 

 (PI. IX., Fig. 15) without any further division of the cleavage 

 cells. This peculiarity is due to a change in the form of the 

 cells, whereby their long axis becomes really tangential. The 

 cleavage cells now divide very rapidly and it is impossible to 

 distinguish any further regularity in the process of cleavage 

 (Fig. 16). 



Origin of Endoderm. 



When the cleavage cavity reaches its maximum growth, the 

 embryo consists of a large spherical blastula with its single layer 

 of primitive ectodermal cells, which have about the same thickness 

 throughout ; but some of them now begin to enlarge, so that they 

 come to project into the cleavage cavity (PI. X., Fig. 17), and 

 instead of dividing parallel to the surface begin to divide at right 

 angles to it. The inner ends of the divided cells become free in 

 the cleavage cavity and give rise to the endoderm. A section 

 passing through the equatorial plane (PI. IX., Fig. 18) shows the 

 cells not only dividing radially but also tangentially. This proc- 

 ess continues until the cleavage cavity becomes filled with cells 



