300 Thos. II. Montgomery. 



by the deposition of the yolk the cytoplasm becomes forced into those 

 positions not occupied by the yolk. Accordingly, in the stage of Fig. 

 1 cytoplasm is probably present everywhere between yolk spheres, 

 though for the most part in minimal amount. The peripheral blas- 

 tema is present throughout cleavage and does not become marked into 

 cell territories until all the cleavage cells have moved into it; the 

 areas it exhibits on surface views are simply impressions due to the 

 underlying yolk spherules. 



The early four-cell stage is presented in Fig. 2, with the nuclei a 

 little further apart from the centre of the egg. Fig. 3 shows an eight- 

 cell stage (71/2 hours) ; this drawing is a combination from a series 

 of sections, and only the superficial blastema (Cyt), the eight cleav- 

 age cells and certain supernumerary sperm nuclei (Sp) are shown; 

 those cells in the upper half of the egg are shaded, those of the lower 

 hemisphere shown in profile only. Fig. 4 represents a portion of one 

 section of the same egg. Polyspermy is frequent, as I described in my 

 account of the fertilization, though only one sperm nucleus passes 

 to the center of the egg to become a male pronucleus, the others al- 

 ways remaining at the periphery. Later than the stage when the 

 cleavage nuclei have reached the surface I have been unable to distin- 

 guish such accessory sperm nuclei with certainty, and cannot tell 

 ^\^hether they take part in the formation of the blastoderm. 



The sixteen-cell stage occurs in eggs aged from 8 to IOV2 hours after 

 oviposition. All the nuclei of such a stage, each in the metaphase 

 of mitosis, are shown in Fig. 5, a reconstruction from a series of sec- 

 tions (like Fig. 3). The cleavage cells have moved still nearer the 

 surface, as shown best in Fig. 6, apportion of a single section. I ha^'e 

 not attempted to draw the separate yolk globules from this stage on, 

 for after the action of the fixative employed the yolk generally coagu- 

 lates into a more or less homogeneous mass. Still another action of 

 this fixative is to be seen from this stage on, the formation of a central 

 fluid cavity {Cav., Fig. 6), but since this is found at only a particu- 

 lar stage it is probably not an artifact. But, though, after this method 

 of preservation the yolk often ,becomes greatly changed from the 

 natural condition, the cellular structures are shown with the greater 

 clearness and fidelity. As the cleavage progresses the cells on near- 



