52 Bashford Dean Memorial Voluyne 



FIRST CLEAVAGE STAGE 



The germinal hillock previously referred to (Text-figure 3) is now shown in lateral 

 surface view in Plate I, figure 1, drawn with the shell removed. It is a nipple-shaped 

 cone of densely aggregated protoplasm with the sides sbping gently at the base and is 

 continuous with a pellicle of cytoplasm surrounding the remainder of the egg. This 

 structure is the protoplasmic cap previously referred to as found at the animal pole of the 

 elliptical egg. The first cleavage furrow is vertical. It appears on top of the hillock and 

 according to Dean lies in the plane of the micropylar canal. In Plate I, figure 1, the plane 

 of first cleavage is somewhat oblique to the observer, the egg being rotated sHghtly to 

 the left. 



The first cleavage furrow is further shown in polar view in Plate I, figures 2 and 3. 

 In these drawings the region traversed by the first cleavage furrow is the summit of the 

 germinal hillock shown in Plate I, figure 1, whose gently sloping base is here represented 

 by the more softly shaded area which appears like a halo in the background. Plate I, 

 figure 2, apparently shows^the first cleavage furrow in an early stage of development while 

 Plate I, figure 3, represents the same cleavage furrow in a later stage. We deduce this 

 from our observations of other vertebrate eggs studied in the living condition, in which 

 the first cleavage furrow is in the beginning broad in the region of the animal pole but later 

 becomes constricted in this region but broad and shallow at the ends. As shown in the 

 figures, this cleavage plane does not extend marginally to the base of the hillock. These 

 conclusions drawn from the figures and based on the study of other and living eggs, are 

 definitely confirmed by Dean who writes "In a stage of late first cleavage which my 

 material has afforded the blastomeres are drawn closely together at the sides of the 

 cleavage plane [furrow?] ; but even here this has not extended quite to the rim of the 

 germinal cap." 



SECOND CLEAVAGE STAGE 



The second plane of cleavage is also vertical and is approximately at right angles to 

 the first. The resulting blastomeres are of nearly equal si2;e. In Plate I, figure 4, showing 

 the polar hillock in lateral view, is portrayed an early phase of the four-celled stage. Figure 

 5 on this plate shows a polar view of an egg in practically the same stage. Plate I, figure 6, 

 gives a polar view of an egg in a slightly later four-celled stage, in which the furrows are 

 broad at their outer extremities but slightly constricted in mid-course. At the center of 

 the germinal hillock there is a slight depression. 



Figure 7, plate I, is a lateral view of a late phase of second cleavage in which the 

 furrows are broad and shallow at their outer and lower extremities and the inner angles 

 of the blastomeres have withdrawn slightly from contact with one another. By compari- 

 son with Figure 1, plate I, we note that in the second cleavage stage the germinal hillock 

 is no longer nipple-shaped but is now dome-shaped. Figure 8 is a polar view of the same 

 stage of cleavage as that protrayed in Figure 7, and according to Dean's notes is drawn from 

 the same egg. Here is shown the same widening at the outer ends of the furrows and the 

 same retraction of the blastomeres where the planes of cleavage cross leaving a pit which 



