1 2 Edmund B. JVilson. 



germ-nuclei takes place. At the period shortly preceding the 

 first cleavage, when the upper disc has been replaced by the very 

 broad ectoplasmic thickening described above, the lower proto- 

 plasmic area, as seen in surface views of total preparations, varies 

 a good deal in appearance in different individuals, being some- 

 times rounded and fairly well circumscribed, sometimes irregular, 

 or even broken up so as to present a mottled appearance. 



The first cleavage, which occurs about thirty minutes after the 

 extrusion of the second polar body, is characterized by a trefoil 

 stage, like that occuring in many gasteropods, lamellibranchs 

 and annelids (Figs. 5, 6). Exactly surrounding the lower pole 

 is formed, by a horizontal constriction, a large lobe, into which 

 passes the whole of the lower white polar area, and M^hich, like 

 the area itself, appears pure white in the living object. Since 

 the surface of the lobe is much larger than that of the original 

 lower polar area from which it arises, it is evident that material 

 from the interior of the egg must How into the lobe as it form.s. 

 Vertical sections of the egg as the polar lobe begins to form show 

 somewhat varying appearances, due in part to differences in the 

 plane of section, but also in part to varying conditions in the 

 protoplasmic area itself. The rather small cleavage-figure, at 

 this period entirely surrounded by deutoplasm, lies in late ana- 

 phase or early telophase slightly above the centre of the egg. 

 At the lower pole the dense protoplasm of the lower area is now 

 spread out, more or less irregularly, to form a thick peripheral 

 layer that fades away insensibly into the yolk-bearing region. 

 Frequently, as in Fig. 12 {cf. Wheeler's Fig. 46) this thickening 

 appears fairly regular and symmetrical and suggests the ecto- 

 plasmic thickening that precedes the formation of a pseudopod 

 in Amceha; sometimes it is less regular than this, and occasion- 

 ally gives the appearance of an asymmetrical wedge-shaped mass 

 extending into the yolk. As the lobe forms it receives this clear 

 protoplasm, accompanied by an inflow of yolk that seems to in- 

 vade the clear substance more or less; so that in section scattered 

 yolk-granules are found in the lobe and frequently no definite 

 boundary of the clear substance can be distinguished (Fig. 13). 

 In any case it is certain that the whole of the lower protoplasmic 



