TYPES OF CLEAVAGE 299 



development of other vertebrate eggs, it is apparent that the nutritive (tropho- 

 blast) cells are located at one pole, while the formative cells of the embryo 

 are found toward the opposite pole. The latter condition resembles the ar- 

 rangement of formative cells and nutritive substances in teleost and elasmo- 

 branch fishes, in reptiles, birds, and prototherian mammals. This comparison 

 suggests, therefore, that the first cleavage plane in the higher mammals cuts 

 at right angles to the true median axis of the egg (cf. fig. 145A-E). If this 

 is so, the first cleavage furrow should be regarded as latitudinal and almost 

 equatorial, and the two blastomeres should theoretically be arranged as shown 

 in figure 145 A. 



The determination of the animal and vegetal poles of the egg in this group 

 of vertebrates is difficult by any other means than that suggested above. In 

 many lower chordate species the polar bodies act as indicators of the animal 

 pole, for they remain relatively fixed at this pole of the egg (e.g., Styela, 

 Amphioxus, etc.). But in higher mammals the polar bodies "are never sta- 

 tionary, and there is evidently much shifting" (Gregory, 30, relative to the 

 rabbit), although in the two-cell stage, the polar bodies often appear between 

 the two blastomeres at one end. It appears in consequence that the fates of 

 the two blastomeres of the two-cell stage serves as a better criterion of egg 

 symmetry at this time than is afforded by the polocytes. According to this 

 view, the smaller blastomere should be regarded as indicating the animal 

 pole, while the larger blastomere signifies the vegetative pole (fig. 145A). 



With respect to the statements in the previous paragraph, it is well to 

 mention that Nicholas and Hall ('42) reported that two early embryos may 

 be produced by isolating the blastomeres of the two-cell stage in the rat, and 

 one embryo is produced as a result of experimental fusion of two fertilized 

 eggs. These experimental results suggest that the potencies of the two blas- 

 tomeres are not so rigidly determined that two different kinds of development 

 result when the blastomeres are isolated. In normal development, however, 

 it may be that the innate potencies of the two blastomeres are not precisely 

 the same. The ability to regulate and thus compensate for lost substances 

 shown by many different types of early embryonic blastomeres, may explain 

 the production of two early embryos from the separated blastomeres of the 

 two-cell stage. 



The second cleavage divides the larger blastomere into two cells, giving 

 origin to three cells. Then the smaller blastomere divides, forming four cells. 

 Cleavage from this time on becomes irregular, and five-, six-, seven-, eight-, 

 etc., cell stages are formed. 



Segmentation of the higher mammalian egg, therefore, is unique in its 

 cleavage pattern. The synchrony so apparent in the egg of Amphioxus is 

 lacking. Irregularity and individuality is the rule, with the auxiliary or nutritive 

 pole cells dividing faster than those of the formative or animal pole cells. 



Moreover, the blastomeres not only show their apparent independence of 



