TYPES OF CLEAVAGE 



307 



VERTICAL FURROW 



Fig. 150. Cleavage in the egg of Lepidosiren paradoxa. (After Kerr, '09.) (A) Be- 

 ginning of first cleavage, meridional in position. (B) Second cleavage planes, approxi- 

 mately meridional in position. (C) Third cleavage planes vertical in position, demon- 

 strating a typical meroblastic pattern. (D) Early biastula. (E) Late blastula. 



laden macromeres. The blastocoel is small in relation to the size of the egg 

 (Eycleshymer, 1895). 



2) Lepidosiren paradoxa. The egg of the South American lungfish, 

 Lepidosiren paradoxa, measures about 6.5 to 7 mm. in diameter. Cleavage 

 of the egg is complete (i.e., holoblastic), and a relatively large blastocoel is 

 formed. As in Ambystoma, the blastocoel is displaced toward the animal 

 pole. The floor of the blastocoel is formed by the large, yolk-laden macromeres. 



The first two cleavage furrows are approximately meridional (fig. 150A, B). 

 These two furrows are followed by four vertical furrows, which, when com- 

 pleted, form eight blastomeres (fig. 150C). The latter cleavages are subject 

 to much variation. Although cleavage of the egg is complete, a distinct mero- 

 blastic pattern of cleavage is found, composed of two meridional furrows 

 followed by vertical furrowing (see Kerr, '09). 



3) Necturus maculosus. In this species of amphibia the egg is large and 

 its contained yolk is greater than that of Ambystoma. It measures about 5 to 

 6 mm. in diameter. The egg and its envelopes are attached individually by 

 the female beneath the flattened surface of a stone (Bishop, '26). 



Cleavage in this egg proceeds slowly. The first two cleavage furrows tend 

 to be meridional, but variations may occur in different eggs. Sometimes they 

 are more vertical than meridional (fig. 151 A). (See Eycleshymer, '04). The 



