EMBRYOLOGY OF THE SEA LAMPREY 



117 



meridional furrow or a single equatorial division. 

 The meridional type was described by McClure 

 (1893) for P. marinus eggs held at 6°-8° C. (42.8°- 

 46.4° F.) or at room temperature, whereas both 

 types of cleavage were found in this study. The 

 equatorial cleavage was not mentioned bj- Mc- 

 Clure, although it predominated in the present 

 study in all experimental batches, regardless of 

 temperature. 



Embryos formed after meridional cleavage can 

 be distinguished by the flatness at the animal pole. 

 The nuclei are aligned four on each side of the 

 first cleavage plane (fig. 6). In some embrj'os the 

 segmentation cavity can be seen because the 

 embryo splits along the first cleavage plane. 



The flatness of stage 4 embr>-os formed by 

 meridional divisions produces a large perivitelline 

 space which facilitates removal of the fertilization 

 membrane. The same operating space is not 

 encountered again until stage 9. 



The embryos formed by equatorial divisions 

 have 4 micromeres resting upon 4 macromeres. 

 In them, the space available for removal of the 

 fertilization membrane is relatively small. 



The end-pouit of this stage is the appearance of 

 the fourth cleavage furrow. 



Stage 5: Sixteen cells (fig. 7) hours 13-15 



Cellular areas: No longer recognizable from 

 external view. 



Size : 1 .0 millimeter. 



Cleavage : Equatorial or meridional, depending 

 upon type in stage 4. Pattern irregular. Com- 

 pleted within less than 2 hours. 



Stage 5 begins with the appearance of the fourth 

 cleavage, the plane of which varies according to 

 the t>T)e of cleavage which formed stage 4; a 

 third -stage equatorial division, near the animal 

 pole, is followed by a fourth meridional division 

 and vice versa. Cleavage irregularity becomes 

 apparent during this stage; embryos composed of 

 9 to 16 cells are found and included in this stage. 



The end-point of stage 5 is reached when the 

 embryo is composed of 17 or more cells. 



Stage 6: Thirty- two cells (fig. 8) hours 16-19 



Size: 1.0 millimeter. 



Cleavage: Random and indeterminate. Com- 

 pleted within 1 hour. 



FinuRE 7. — Several views of stage o, sixteen cells, illus- 

 trating differences in size of animal and vegetal cells. 



Figure 8. — Several polar views of stage 6, thirty-two cells. 



Embryos were assigned to this stage when 17 

 to 32 cells were distmguishable. Cytokinesis at 

 this point, however, became mdeterminable so 

 that the fifth cleavage appeared to take place at 

 random. Cell counts were made on all embryos 

 that could not be identified on the basis of relative 

 cell sizes by comparison with both stages 5 and 7. 

 When animal cells arc compared with animal 

 cells, and vegetal cells with vegetal cells, the cells 

 of stage 6 embryos will be about one-half the size 

 of stage 5 colls and approximately twice the size 

 of stage 7 cells. The relative sizes of the animal- 



