1907.] NATURAL SCIENCES OF rinLADKLPHIA. 52.J 



the case here, since the distrilnitiou of the yolk granules is uniform 

 throughout the egg. 



Wheeler (94) has stated that the impregnation is probably what 

 Whitman (91) has called "hypodermic." I have several times 

 observed animals apparently in copulation. In this act the two 

 animals remain in contact for some time and move about together. 

 Most fre([uently the ventral sides of the animals were in contact. 

 Fertilization is necessarily internal, although the means by which the 

 sperm reach the eggs is not known. 



Two matiu'ation divisions occur after the egg is deposited. The 

 first occurs about one hour after deposition and the second about one 

 hour later. At eacli of these divisions the egg goes through some of the 

 most remarkable contortions (fig. 2). The egg becomes very irregular, 

 and processes occur from all sides. These processes consist of the more 

 fluid substance and contain few yolk granules. In many cases parts 

 of the egg are cut off entirely. In one case observed, the egg was 

 actually cut into two, so that I first mistook it for a two-cell stage, but 

 later these parts fused and it then imderwent normal development. In 

 some cases minute pieces seem to be cut off which do not fuse with the 

 egg, for some time at least, but continue to float about between it and 

 the egg membrane. 



The movement of the egg substance is very slow. It takes about 

 twenty minutes for an egg to pass through such a contortion and 

 regain its normal spherical shape. The phenomenon is the same at each 

 of the two maturation divisions. Similar phenomena have been 

 observed b}- Hallez (79), Goette (S2) and Selenka (81) in other pol)^clad 

 eggs. Such ama'boid movements are also quite common in the eggs 

 of other animals, especially annelids. 



The First Cleava(;e. 



About one hour after the second maturation division the first cleav- 

 age furrow makes its appearance. The spindle for the first cleavage 

 lies near the centre of the egg. The first two blastomeres are not of 

 equal size, although the difference is slight (fig. 4). In order to make 

 certain that there is a recognizable difference, I have made a number 

 of camera drawings both of living and stained eggs. In all cases where 

 it was not evident that the egg was pressed out of shape in its membrane 

 the difference in size is quite easily recognized. 



According to Lang (84) this difference in size of the first two blasto- 

 meres is very constant in polyclads. Lang says (p. 330): "Ich habe 

 diese allerdings wenig auffallende Verschiedenheit in der Grosse 



