72 EDWIN G. CONKLIN 
direction; the micromeres formed from these abnormal macro- 
meres are the same as in normal eggs in which all the macromeres 
are of the same size and contain the same quantity of cytoplasm 
and yolk. In short there is here a form of regulation which leads 
to the formation of normal micromeres from abnormal macro- 
meres, and the exact manner in which this cellular regulation 
takes place is of fundamental importance, and will be discussed 
later. 
Fig. 23 represents an egg similar in many respects to fig. 21, 
but of a later stage. The smaller protoplasmic macromeres 
preserve their original polarity as is shown by the fact that the 
spheres lie between the nuclei and the polar bodies. On the other 
hand each of the large macromeres contains a tetraster; the spin- 
dles are those of the third cleavage. 
Fig. 24 represents an egg of the same type as the preceding, 
after the third cleavage; each macromere has given rise to a 
micromere which is normal in form, position, constitution and 
size, although the macromeres are very abnormal in these regards, 
two of them containing all of the yolk and very little protoplasm, 
and the other two being small and purely protoplasmic. Indeed 
the macromeres 7A and 1D nearly exhausted all the cytoplasm 
which they contained in order to form cytoplasmic micromeres 
of normal size; on the other hand, the size of the micromeres 
1b and Jc is not influenced by the fact that the macromeres from 
which they come are small and are purely protoplasmic. 
Fig. 25 is a drawing of an egg in a slightly older stage than fig. 
24; the large macromeres, 7B and 1C, are giving off the second 
set of micromeres, 2b and 2c, while two of the first set of micro- 
meres, /b and /c, are just beginning to divide. The four small 
cells which lie to the left of the polar bodies are the macromeres 
1A and 1D and the micromeres Ja and Jd; these cells are purely 
protoplasmic and are very small, all four of them being no larger . 
than one of the micromeres, /b or jc, in the other quadrants. 
Nevertheless these minute ‘macromeres’ have each given rise by 
an equal cleavage, to a micromere as large as itself. Although 
these micromeres are much smaller than those in the other quad- 
rants, they are the largest that could be formed from the macro- 
