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CLEAVAGE (SEGMENTATION) AND BLASTULATION 



The vegetal pole is gray in color. The egg membrane is well developed, having 

 a zona radiata and a villous layer. Strands of the villous layer may attach the 

 egg to the stem of a water weed or other structure (fig. 153A). 



The first cleavage plane is meridional and partly cleaves the protoplasmic 

 disc into two parts (fig. 153B). This cleavage furrow passes slowly toward 

 the vegetal pole of the egg. The second cleavage is similar to the first furrow 

 and at right angles to it (fig. 153C). The third cleavage is variable but, in 

 general, consists of two furrows passing in a vertical plane at right angles to 

 the first cleavage furrow (fig. 153D). The fourth set of cleavages is hori- 



SYNT. YTiai NUCLEI 



Fig. 153. Cleavage in the egg of Amia catva. (After Dean, 1896.) (A) Egg mem 

 branes of Amia, showing the filamentous (villous) layer attaching the egg to the stem 

 of a water weed. (B) Second cleavage plane shown cutting through the protoplasmic 

 disc at one pole of the egg. Section made parallel to the first cleavage plane. (C) First 

 and second cleavage planes seen from above. (D) Third cleavage planes are vertical 

 in position as indicated. (E) Fourth cleavage, sectioned in a plane approximately parallel 

 to first (or second) cleavage. (F) Section through protoplasmic disc at eighth cleavage. 

 (G) Blastular stage. Blastocoel is indistinct and scattered between (?) blastomeres of 

 blastoderm. The description given by Whitman and Eycleshymer (1897) does not agree 

 in certain features with the above. 



