744 GORHAM. [Vol. XL 



Owing to these differences in the method of segmentation 

 we have three distinct types of arrangement of the blastomeres 

 in the four-cell stage (PI. XXXVII, Figs. 9, 10, and 11). This 

 variation in the method of cleavage is not due to artificial 

 causes, but takes place under natural conditions in the sea. 

 The various types of cleavage are found in the eggs of a single 

 female, and all finally produce normal embryos. About fifty 

 per cent of the eggs develop according to Type I (PI. XXXVII, 

 Fig. 9), about twenty-five per cent according to Type II 

 (PI. XXXVII, Fig. 10), and the remaining twenty-five per cent 

 according to Type III (Pi. XXXVII, Fig. 11). 



One is not justified in claiming that certain blastomeres of 

 eggs of different types of segmentation are equivalent, for 

 there are no distinguishing marks whereby the blastomeres can 

 be identified. The eggs are all regular in outline, and the 

 several blastomeres do not vary in size or color, and in Types 

 I and II it is impossible to distinguish one apical or one lateral 

 cell from the other, and in Type III all four blastomeres are 

 exactly alike. 



Third Cleavage. 



Type I. — After the egg is completely divided into four 

 blastomeres the nuclei again divide in such a manner that their 

 spindles are all parallel to each other, and perpendicular to the 

 major axis of the ^%% (PI. XXXVII, Fig. 1 2). The third cleavage- 

 plane is meridianal, and cuts the four blastomeres at right 

 angles to the first and second planes (PI. XXXVII, Fig. 12). 



Type II. — The third cleavage in this type is essentially the 

 same as in the previous type, but the cells derived from the 

 lateral blastomeres of the four-cell stage are in contact on both 

 sides of the egg (PI. XXXVII, Fig. 13). 



Type III — In this type the third process of cleavage results 

 in the formation of four spindles, of which the two in each 

 half of the egg are parallel to each other, but perpendicular to 

 the two in the other half of the egg (PI. XXXVII, Fig. 14). 

 The four blastomeres are then divided meridianally, the new 

 planes appearing as direct continuations of the planes of the 

 second cleavage (PI. XXXVII, Fig. 14). The eight blastomeres 



