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 ; the 

 mouth breaks through at a later period upon the ventral surface of the 

 embryo. 



V. CLEAVAGE. 



Introductory. 



The cleavage of the egg 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 the one-cell to the sixteen- and occasion- 

 ally the tweuty-four-cell stage. By six o'clock 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 a few degrees above freezing, and 

 were found to have just reached that stage at 8.30 the 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 the 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 



