362 THEOPHILUS S. PAINTER 



For the sake of clearness, the course of the different types of 

 eggs will be followed separately through later cleavage in the 

 following description. These will be taken up in the following 

 order: (a) The effects of the abnormal positions which the Si 

 derivatives take in cleavage, (b) The result of the unequal 

 distribution of the chromatin between the A and B blastomeres. 

 (c) The fate of the tetraster eggs. 



Among the treated eggs there is always a certain per cent which 

 are perfectly normal. The 4-cell stage, such as shown in figure 5, 

 is followed by the division of the A and B blastomeres in a plane 

 approximately at right angles to the plane of the paper upon 

 which the drawing is given (compare with fig. G). Following 

 this, the Po and EMSt cells divide in the median plane of the 

 embryo (compare with fig. 1), and throughout the later cleav- 

 age, the analyses with models show that the normal develop- 

 ment is continued. 



The development of embryos in which the A and B blastomeres 

 occupy abnormal positions in the 4-cell stage, may be followed 

 with ease up to the time when the Si derivatives number 10 cells. 

 From this point on, such eggs are not be to distinguished from 

 normal embryos. Figure 4 shows a typical case of the positions 

 which the A and B cell take. In figure 10, we see these two 

 cells dividing. The diminution process is taking place normally, 

 but the planes which the dividing cells occupy, instead of being 

 parallel (compare with the normal as shown in fig. G) are at 

 right angles to each other. The result of such a division is shown 

 in figure 11. In the egg shown in figure 12, the ectodermal cells 

 are eight in number and the EMSt cell has divided in the median 

 plane of the embryo. The Po cell is in the equatorial plate phase 

 of division. The elongated chromosomes characteristic of the 

 primordial germ cell are clearly seen. In figure 13, w^e see a some- 

 what later stage. Both the Po and EMSt blastomeres have 

 divided. It is especially to be noted that the MSt blastomere 

 does not lie in the median plane of the embryo (the EMSt cell 

 divides into an E and an MSt cell; these normally lie in the me- 

 dian plane of the embryo, compare with fig. 1). It is very 

 rare that we find the EMSt cell dividing in any plane but the 



