EMBRYOLOGY OF THE SEA LAMPREY 



115 



within the cytoplasm surrounding the nucleus 

 and the remaining cytoplasm. The yolk of the 

 animal and the vegetal areas differs in that the 

 yolk platelets within the vitelline area are much 

 larger than those in the area around the nucleus. 



The depression at the animal pole seen in stage 

 persists through this stage. The first indication 

 of significant morphological change is a noticeable 

 deepening of this depression which will remain 

 until about the first hour after fertilization. 



Three distinct external areas indicate the 

 presence of the three internal areas: nucleus; 

 animal-pole cytoplasm; and vegetal-hemisphere 

 cytoplasm. Below the nucleus the cytoplasmic 

 area is demarcated by a band extending approxi- 

 mately one-third the distance down the animal- 

 vegetal axis. The remainder of the egg consists 

 of cytoplasm heavily laden with j'olk. Each of 

 the areas of the egg is visible both externally and 

 internally as early as the unfertilized egg and up 

 to the initiation of first cleavage. 



Shortly after the animal-pole depression deepens, 

 the fertilization membrane appears. 



Stage 2: Two cells (fig. 3) hours 2-8 



Cellular areas : Visible in the daughter cells. 

 Size: 1.0 ±0.2 millimeters. 



Cleavage: First furrow appears. Prominent 

 peaks in daughter cells. Holoblastic. Com- 

 pleted within 5-6 hours. 



The external topography of the 2-celI stage is 

 comparable to that of the zygote in that the nu- 

 clear, and the animal- and vegetal-hemisphere 

 cytoplasmic areas are readily visible in both 

 daughter cells immediately after reconstitution 

 of the nucleus (fig. 3). 



This stage begins when the animal pole of the 

 zygote begins to furrow and dimple in preparation 

 for first cleavage, and ends at the beginning of the 

 second cleavage furrow and dimple at the animal 

 pole. Cleavage is total, usually slightly unecpial 

 (fig. 4) but occasionally (less than 1 percent) 

 grossly unequal. As mentioned above, cleavage 

 begins as a small furrow; a slight uprising of the 

 cell membrane lateral to the furrow produces the 

 dimpled effect. As cytokinesis progresses the cell 

 membrane expands to a greater and greater extent 

 while the fertilization membrane remains un- 

 changed. The cell membrane continues to expand 

 until the cell reaches the stage seen in figure 3, 

 where the expanded cell membrane can be seen as 

 twin peaks lateral to and above the furrow. As 

 the cleavage furrow progresses meridionally over 



Fkutre 3. — External views of stage 2, two cells, showing the expanded cell meinl)raiie (right side) and daughter cells 



showing the cellular areas. 

 566128 0—61 2 



