140 MARY J. GUTHRIE AND HOPE HIBBARD. 



lucida sketches of the cleavage stages which he made from 

 living eggs. While it is possible to make out these stages in the 

 preserved specimens, they are much clearer in the living eggs; 

 and his sketches have been used to illustrate the segmentation 

 changes. 



EARLY CLEAVAGE. 



The Toxopneitstes egg has been found especially favorable for 

 many kinds of study because of its transparency and beautiful 

 clearness of structure, when fixed as well as while alive. 



As this paper deals with a definitely limited phase of develop- 

 ment it seems best not to include a detailed description of the 

 egg, or its jelly layer, or of the manner of formation of the 

 fertilization membrane, but to pass at once to the consideration 

 of cleavage. 



The first cleavage of the Toxopneustes egg is vertical and 

 results in two equal cells being formed about forty minutes after 

 insemination (Figs. I, 2, 3 and 4). Tennent (1911) gave the 

 characteristic rhythm for Toxopneustes cleavage at 28 C., and 

 the time intervals indicated are from his paper. This rhythm 

 may be altered by changes in temperature as has been shown 

 for example by Loeb and Chamberlain (1915) who worked with 

 Arbacia eggs. They found a close correlation between the tem- 

 perature of the water in which the eggs were kept and the period 

 of time elapsing between insemination and the first cleavage, 

 and were, therefore, able to calculate the temperature coefficient 

 for cell division at varying temperatures. 



The second cleavage, about one hour and ten minutes after 

 insemination, is also vertical, and divides the egg into four equal 

 cells which are shown in polar view in Fig. 5. After one hour 

 and thirty-five minutes, the third cleavage furrow coming in 

 horizontally divides the egg into eight equal cells (Figs. 6 and 7). 

 This is the last cleavage in which all of the cells are equally 

 divided. 



A twelve cell stage is formed by a horizontal division which 

 cuts off four small cells, the first set of micromeres, at the vegetal 

 pole. Practically coincident with this, the four cells at the 

 animal pole are divided by vertical furrows into eight. In the 

 sixteen cell stage there are therefore, three different kinds of 



