44 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
induced by the environment of the egg. The primary mesoblast divides 
bilaterally, ultimately sinks below the general level, and forms two 
bilaterally placed mesodermal bands extending anteriorly. Their forma- 
tion precedes and accompanies gastrulation, no lumina appearing at any 
time within them. The blastopore is at first broad and shallow, but it 
gradually deepens at the anterior end, and disappears from the posterior 
margin anteriorly, forming an elliptical pit on the median ventral surface. 
By a rapid growth in the latero-anterior lips of this pit, accompanied by 
an accumulation of mesoderm in these regions and a general readjust- 
ment of the axes of the embryo, the opening leading into the archenteron 
assumes a position at the former posterior margin of the blastopore. 
This remnant of the blastopore comes to lie in the anal region; tho 
mouth breaks through at a later period upon the ventral surface of the 
embryo. 
V. CLEAVAGE. 
Introductory. 
The cleavage of the ogg of Limax takes place with considerable rapidity. 
The eggs are generally laid, in captivity, during the night, and in the 
morning one finds stages from tho one-cell to the sixteen- and occasion- 
ally thé twenty-four-cell stage. By six o'elock in the evening these 
eggs have reached the stages of twenty-four to forty or more cells. 
Gastrulation begins during the second day, and is completed on the 
third day. There is, however, much variation even in a lot of eggs 
found in one mass, and evidently laid by one individual. These dif- 
ferences may possibly be due to differences in the time of fertiliza- 
tion. Temperature exercises a profound influence on the rate of cleav- 
age. Eggs about to pass into the twenty-four-cell stage at 6 P. M. were 
placed over night in a temperature à few degrees above freezing, and 
were found to have just reached that stage at 8.30 tho next morn- 
ing, and, though restored to the temperature of the laboratory, they did 
not progress to the next cleavage until 2 p.m. There are a few “ stages ” 
in tho cleavage that are well marked, i. e. periods of apparent inactivity 
in which the egg continues for some time. These are the the two-, four-, 
eight-, sixteen-, twenty-four-, forty-four-, and sixty-cell stages. The 
periods alternating with these are marked by mitotic conditions in all 
or a part of the cells of the egg. 
The animal pole of the mature and undivided egg is marked by the 
presence of two polar globules. These generally differ in size, the more 
