578 HAROLD HEATH, 



of the egg itself. On the other hand a delicate viteline mem- 

 brane is preseot in the fertilized egg and also, I believe, in the un- 

 fertilized ones. 



The individuals that laid in the aquarium were brought from a 

 point three or four miles distant from the laboratory. In their trans- 

 portation the water was changed several times. The eggs subsequently 

 laid in the aquarium with relatively few exceptions did not develop, 

 from which 1 conclude that the sperm were in the sea water and 

 were removed in changing it, whence it probably follows fertilization 

 takes place outside of the body ^). This method of fertilization is 

 described by Metcalp ('93) for Chiton marmoratus and C. squamosus, 

 and I have noticed it in the case of Mopalia lignosa. 



Some of the specimens were killed while laying, and the eggs in 

 the ovary showed what I believe to be the first maturation spindle 

 in process of formation. It is surrounded by a small protoplasmic 

 area while the remainder of the egg is uniformly filled with yolk. 

 The spindle is of very small size being but about one ninth the 

 diameter of the egg. 



II. The Primary Cleayages. 



1. First Cleavage, 2 cells (Fig. 1). 



Basing my observations on those Chiton that laid in the aquarium, 

 two hours and ten minutes elapse from fertilization to the first 

 cleavage. After this period a slight flattening appears at the animal 

 pole which is followed by the formation of a furrow rapidly encircling 

 the egg. This divides the ovum in about ten minutes. At this time 

 the two blastomeres are somewhat spherical but they soon become so 

 pressed together that they are almost perfect hemispheres; indeed in 

 the living egg it is often difficult to make out the boundary between 

 the two. By this division two cells result one of which is larger than 

 the other, but this diflerence in size is usually very slight, and often 

 it is impossible to detect any inequality. 



I have taken much pains to attempt to determine whether this 

 ditference in the masses of these blastomeres is correlated with the 

 more rapid development of the posterior quadrants, but have found 

 no constant feature which would enable me to orient the egg in these 



1) The eggs that were fertilized developed normally, proving that 

 the environment was normal. 



