IIG 'T. Thomas Patterson. 



half shows exact regularity, there being just eight cells, but the 

 jDosterior half is slightly irregular, with nine cells. 



There are no accessory cleavages in this stage, but a large num- 

 ber of short radial furrows lie just inside the margin of the primary 

 area (Fig. 20). These furrows can be seen in the living egg, but 

 are brought out more clearly in osmic acid preparations. Under 

 the higher power of the microscope they are seen to be due not so 

 much to the formation of furrows as to the arrangement of the 

 granules. In the median lines they are composed of fine granules, 

 while to either side are several rows of larger granules. Occasion- 

 ally one can detect a membrane, which appears as a delicate thread 

 running through the median streak of fine granules. 



At first sight it might seem that these furrows represent an abun- 

 dant accessory cleavage, but such is not the case. In the first 

 place, the furrows are situated entirely within the primary area, 

 while the accessory furrows lie without the area. In the second 

 place, there are no nuclei directly associated with these short fur- 

 rows. They are simply the beginnings of the peripheral extensions 

 of the primary cleavage furrows ; for it will be noted that for the 

 most part they lie either in the same radial planes with the primary 

 furrows (marginal), or in j^ositions where the next divisions of 

 the marginal cells will soon occur. This interpretation is fully 

 substantiated when slightly later stages were studied, when it was 

 found that not only are the short furrows greatly diminished in num- 

 bers, but that the primary furrows now reach the margin of the 

 primary area (Fig. 15), and at the same time the marginal cells are 

 practically double in numbers. 



These short furrows do not occur in all blastoderms, but when 

 they do appear it is at the sixteen-celled stage, though they may last 

 until comparatively late cleavage stages (Fig. 30). In significance 

 these furrows indicate that the cytoplasmic division of the marginal 

 cells is felt at the margin of the primary area earlier than in re- 

 gions lying somewhat more centrally; and the attempted division at 

 the margin probably immediately follows that of the marginal 

 cell nucleus, while the intervening space awaits the approach of the 

 central portion of the primary cleavage membranes. 



