MARY J. GUTHRIE AND HOPE HIBBARD. 



TABLE SHOWING SEQUENCE OF CLEAVAGE. 



1 It will be noticed that there are two columns headed i h. 50 m. and two headed 

 2 h. 55 m. This is for the purpose of indicating that the formation of the micro- 

 meres is completed before complete division of the four small macromeres, about 

 one hour and fifty minutes after insemination, and that the division of the sixteen 

 small macromeres is completed before that of the upper eight large macromeres, 

 two hours and fifty-five minutes after insemination. The duration of an actual 

 twelve cell and of an actual fifty-six-cell stage is short. 



four large ones. Immediately after this segmentation or, though 

 more seldom, before it is accomplished, the four lower segments 

 are divided by a vertical plane into eight almost equal spheres, 

 which become arranged in a more or less irregular manner in 

 two rows." 



Further, in Echinocyamus, two hours and thirty-five minutes 

 after fecundation, the large macromeres are divided vertically, 

 the small macromeres are cut horizontally, and after fifteen or 

 twenty minutes " the four micromeres at the animal pole become 

 parted by horizontal planes into eight, four of which are smaller, 

 contain a clearer protoplasm and, as they lie in the same plane, 

 constitute the animal pole itself; the four remaining ones have 

 their place somewhat outside and below r ." It will be noticed 

 that the small macromeres are divided horizontally in Echino- 

 cyamus, while in Toxopneustes the division is vertical. 



Theel fell into error concerning the micromeres probably 

 because he assumed that they w r ere homologous with the micro- 

 meres of flatworms, annelids and molluscs. Boveri ('01) in his 

 study of the egg of Strongylocentrotus livid us, demonstrated that 

 it is at the vegetal pole that the micromeres are separated at the 

 twelve-cell stage. 



FORMATION OF MESENCHYME. 



Very early in the development of the Toxopneustes blastula 

 the cells w r hich are to give rise to the future mesenchyme can 



