l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.98 



nucleus stages respectively. The details of the differentiations of these 

 various stages deserve special consideration. 



Mention has been made of the fact that at the last or seventh 

 cleavage division, which occurs during the migration of the pre- 

 blastema nuclei to the periplasm, the mitotic spindles are arranged 

 parallel to the egg's surface, thus insuring that the daughter nuclei will 

 be equally distant from the periplasm when they are formed. All the 

 division planes of the three subsequent nuclear divisions likewise are 

 at right angles to the egg's surface. This phenomenon serves to retain 

 the single-layered arrangement of the nuclei in all the blastema 

 substages. 



Immediately following the eighth nuclear division or when the egg 

 is in the 256 nucleus substage of the blastema, the two daughter nuclei 

 of any one parental nucleus lie close together. Soon, however, they 

 draw away from each other until they are approximately equidistant 

 from the adjacent nuclei and from each other (pi. 1, fig. 5; pi. 4, 

 fig. 40). The egg is then ready for the ninth nuclear division. This, 

 and the tenth division, take place in the same manner as the eighth, 

 although the pairs of daughter nuclei are less apparent owing to the 

 greater number and resultant crowding of the nuclei in the periplasm. 

 The third blastema substage, following the ninth nuclear division, is 

 shown in figures 3, 10, and 11 (pi. 1) and in figures 41 and 42 (pi. 4). 

 Figures 8 (pi. 1) and 49 (pi. 6) represent the fourth blastema sub- 

 stage. This follows the tenth nuclear division and corresponds to the 

 1024 nucleus stage. 



In the fourth blastema substage, all of the first true blastodermic 

 nuclei are accounted for. All that remains to transform the blastema 

 into the blastoderm is the separation of the individual nuclei with their 

 adjacent cytoplasm into distinct cell territories. The cells delineated 

 in this manner are the first true cells of the developing egg. Some 120 

 of them are present in a median longitudinal section of the primary 

 blastodermic stage or the first substage of the blastula (pi. 1, figs. 7, 9 ; 

 pi. 5, fig. 48). The discussion of the second and third blastula sub- 

 stages is postponed to the section on the formation of the germ band. 



The cytoplasm of the blastoderm has two sources. Part of it is 

 derived from the thin periplasmic layer, and the remainder has been 

 carried into this region by the immigrating cleavage nuclei. The first 

 indication of the delimitation of the cell territories of the blastoderm 

 is the appearance of slightly rounded swellings at the surface of the 

 egg. These swellings are opposite the individual peripheral nuclei. 

 Almost immediately the lateral cell walls begin to form. They begin 

 at the surface and gradually extend inward. The nuclei do not lie in 



