CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 243 



into the thin layer of peripheral protoplasm, these first blastomeres 



as well as those that follow (Figs. 107 to 109) are not distinctly 

 bounded externally but fade away into the peripheral protoplasm. 

 This is still the case with the peripheral cleavage-cells of later stages 

 (Figs. 108 C to 111). Yialleton d urines these as blastocones as 

 opposed to the hlastcmieres. 



A further step in the cleavage is marked by the breaking up of 

 each of the four segments now present into two new segments (Figs. 



Fig. 109. — Germ-disc of Sepia officinalis at the eight-celled stage i.l) ami at two later 

 stages (/,' and '') (alter Viai.LETon). The germ-discs are placed in such a way that 

 the anterior side is directed upwards, ami the posterior side downwards. I, left side 

 r, right side ; l-V, directions of the first live meridional planes of cleavage. 



109 A and 108 B). Of these new furrows (///' and III") only the 

 anterior ones (III) make an angle of 45° with the median plane and 

 therefore cut the two anterior segments into almost equal halves. 

 The two posterior furrows (III") run somewhat parallel to the median 

 plane (Figs. 108 B and 109 A), and it thus happens that here, at the 

 posterior part of the germ-disc, two narrow segments bounded by 

 parallel sides are cut oft'. Through this course of cleavage, concerning 



