46 bulletin: museum of comparative zoology. 



B. Discussion of Cleavage. 



Second Generation*. First Cleavage Plane. Two Cells. 



Plate I. Fig. 14. 



It should be noted that the phrase " generation of cells " is used in its 

 strict genealogical meaning, and not in the sense in which Fol ('75, '76), 

 Blochmann ('81), McMurrich ('80), and Heymons ('93) have used the 

 phrases "generation of micromeres " and " generation of cells." The pro- 

 cesses of maturation, fecundation, and the formation of the first cleavage 

 furrow have been described in detail in Dr. Mark's classic work upon 

 Limax campestris. His published work was not carried beyond this 

 stage, though he had continued his researches far into the later stages 

 of development. The appearance of Fol's work ('80), which dealt 

 largely with the embryology of Limax, and the pressure of other duties, 

 have interfered with the completion and publication of his studies. Dr. 

 Mark has vei'y kindly placed his numerous drawings and careful notes in 

 my hands, and they have been of invaluable assistance to me in my work. 

 I shall not attempt to add to his complete description of the formation 

 of the first cleavage plane, but shall begin my woi'k with the stage 

 represented in Plate I. Fig. 14. This is a lateral view of an egg which 

 has just completed the first division. Warneck ('50) has stated that 

 in Limax and Lymnceus this plane is oblique to the axis of elongation, 

 instead of transverse to it, and has distinctly said that this conclusion 

 was not based upon a deceptive orientation of the egg. Fol ('75) has 

 described a similar occurrence in Cymbulia. I have found no evidence 

 that in the least confirms this statement of Warneck's. At the stage 

 shown in Figui'e 14 the two nuclei lie close to the approximated sur- 

 faces of the blastomei'es, at a level about midway between the animal and 

 vegetative poles ; they are still quite small, and have only a very deli- 

 cate membrane. Each has an elongated oval outline, with the long axis 

 extending toward the astrocoel of the cell in which it lies. Their 

 position indicates that in the progress of the cleavage furrow toward the 

 vegetative pole the nuclei (daughter segments) were in some way car- 

 ried downward toward that pole. Mark ('81) has described such a pro- 

 cedure in the eggs of Limax campestris. There are a number of deeply 

 staining granulations in the peripheral part of the cell adjacent to the 

 nuclei, which probably constitute the remnant of the cell plate ; there 

 is thus every indication of recent cell division. 



The astrocoels appear as clear areas, almost as large as the nuclei, 

 containing a few scattered deeply staining granules. These clear areas 



