634 L- R- t'AßY, 



In the first segmentation division (Fig. 44), the spindle becomes 

 excentric in position so that the equatorial plate lies to one side of 

 the chief axis of the egg. As the result of this division there are 

 formed two small cells of unequal size. The diameter of the larger 

 one exceeds that of the smaller one by an amount nearly equal to 

 the diameter of the polar body. At the second division another small 

 cell is cut off from the larger one. There are thus formed two small 

 cells (primary micromeres) and one large cell (macromere) (Fig. 47). 

 In the next few divisions the micromeres alone take part, while the 

 macromere remains in a resting condition until it has been almost 

 completely overgrown by the cap of micromeres. The division of 

 the micromeres usually takes place simultaneously, so that a stage 

 in which there are five cells (Fig. 49) follows one in which there 

 are three. In some instances, however, the sections show embrj'os 

 made up of four cells as in Fig. 48. 



Up to this stage in the segmentation the outlines of the cells 

 remain tolerably distinct, so that the endoderm cell (macromere) can 

 clearly distinguished from the ectoderm cells (micromeres) by its 

 size. In later stages the outlines of the cells become lost so that 

 the whole embryo makes up a syncytium. In many instances, even 

 after the cell outlines have dissappeared, a line separating the cells 

 which have come from the primary macromere (endoderm) from 

 the enclosing mass of cells (ectoderm) remains distinct up to the 

 time when the endoderm cells become arranged to form the primor- 

 dium of the gut (Figs. 53, 54 and 56). 



The formation of the investing membrane. 



When the embryo has reached the nine cell stage two of the 

 micromeres become distinguishable from the others by the more 

 hyaline appearence of their nuclei. These two cells are pushed out 

 from among the others so that they lie on the surface of the mass 

 of cells. Their cytoplasm becomes flattened out to form an exceedingly 

 thin layer over the surface of the remaining cells. The investing 

 cells divide once after they have come to lie on the surface of the 

 embryo, so that when completed the investing membrane consists 

 of four cells. The nuclei of these cells remain for some time as 

 prominent protuberances on the surface of the embryo (Fig. 50). 

 Later they become flattened out so that they are hardly more than 

 twice the thickness of the cytoplasmic part of the membrane. Ulti- 

 mately they seem to disappear entirely, as sections through later 



