420 GEORGE W. TANNREUTHER 



In the four-cell embryo, the large cell, D, is posterior, B 

 anterior A left, and C right. The ectoderm is separated from the 

 four macromeres by a series of three cleavages. The three quar- 

 tettes of ectomeres share in the formation of the ectomesoderm. 

 The macromere 3D gives rise to all of the entoderm, the repro- 

 ductive bodies, and some of their directly associated muscle 

 fibers. The macromeres 3A, 3B, and 3C, do not invaginate 

 with 3D, but remain on the surface in the region of the blasto- 

 pore and, by a rapid proliferation of cells, give rise to the major 

 portion of the temporary foot, which is absorbed before birth. 



Three kinds of eggs are formed: The female-producing and 

 the male-producing, both of which develop parthenogenetically, 

 and the thin- and thick-shelled resting eggs, which are fertilized 

 and always produce females. There are two kinds of adult 

 females, one produces females parthenogenetically and the other, 

 males parthenogentically and resting eggs. Males and females 

 or resting eggs and females are never produced by the same 

 parent. The male and resting eggs have two polar bodies. 

 When conditions are favorable, it requires about seventeen hours 

 from the time of the formation of the polar body to the time of 

 birth of the parthenogenetically produced males and females. 

 The males and females are sexually mature at birth. The female 

 increases four to eight times its size at time of birth. The males 

 do not increase in size after birth, but perish almost immediately 

 after copulation. Very few free-swimming males are found at 

 any one time in the culture. The males are structurally de- 

 generate. The development of the males and females is similar 

 up to the late period in embryonic life. The cells of the stomach 

 and the invaginated pharynx become continuous, but the 

 oesophagus is never formed. 



Gastrulation is epibolic. A central cavity is formed, which 

 begins at the close of the formation of the third quartette of 

 micromeres. The cavity is due to the shortening of the cells in 

 the epithelial ectoderm, and is later occupied by the large cell, 

 3D. The gastrula is composed of a single outer epithelial layer, 

 enclosing 3D, d^, and d^ with their later derivatives. 



