FORCE WHICH CAUSES THE BLASTOMERES TO ADHERE 



327 



derm itself two types of cells may be observed, namely, a superficial ecto- 

 dermal cell and a more deeply situated, somewhat vacuolated, smaller ento- 

 dermal cell (fig. 175B). (For later stages of blastulation, see chapter 7.) 



e. Cleavage in the California Hagfish, Polistotrema (Bdellostoma) stouti 



The California hagfish spawns an egg which is strongly telolecithal. The 

 germinal disc (blastodisc) is situated immediately below the egg membrane 

 at one end of the egg, adjacent to the micropyle and the anchor filaments 

 (fig. 162A). Cleavage begins in this disc, and the enlarging blastoderm slowly 

 creeps downward to envelop the massive yolk material. The freshly laid egg 

 measures about 29 mm. by 14 mm., including the shell. Without the shell, 

 the egg is about 22 mm. by 10 mm. and is rounded at each end (Dean, 1899). 



The first two cleavage planes may be regarded as meridional (or vertical) 

 (fig. 162B). The third cleavage appears to be a mixture of vertical and hori- 

 zontal (latitudinal) cleavages, with the former predominating (fig. 162D, E). 

 Cleavage from this time on becomes irregular, and a typical meroblastic blasto- 

 derm soon is attained with central and marginal cells (fig. 162F). 



C. What is the Force Which Causes the Blastomeres to Adhere 

 Together During Early Cleavage? 



A question naturally arises concerning the force which makes the blasto- 

 meres of most chordates adhere to one another during the early cleavage 



ANCHOR 

 FILAMENT 



MIC ROP YL 



BL A STODE 



( G ER M I N A 



PLASM) 



BLASTODERM 



Fig. 162. Egg and cleavage in the marine lamprey, Polistotrema (Bdellostoma) stouti. 

 After Dean, 1899.) (A) Animal pole end of the egg. (B) Surface view of blasto- 

 dermic hillock, showing first cleavage furrow. (C) Same, second cleavage. (D) Third 

 cleavages. (E, F) Later cleavages, strongly irregular. (G) Egg with shell removed. 



