68 bulletin: museum of comparative zoology. 



The methods employed have been given at length, because it is be- 

 lieved that the results obtained, which difter widely from those of earlier 

 workers, are due largely to the successful making of transparent prepa- 

 rations of entire eggs. In examining the figures given by previous 

 workers it is evident that none of them had the advantage of such 

 preparations, and consequently none of them were able to follow accu- 

 rately the history of the nuclei, which is very important for the determi- 

 nation of cell-lineage. 



IV. Maturation and Fertilization. The Unsegmented Ovum. 



In agreement with the observations of Weismann und Ischikawa ('88), 

 ef^'^s taken from the oviducts were found to contain the first maturation 

 spindle. Owing to mutual pressure, there is great distortion of the eggs 

 in the oviducts, but when artificially liberated into sea water they quickly 

 assume a spherical form. The separation of the first polar cell takes 

 place at about the time when the eggs leave the oviducts. Soon after 

 this the formation of the vitelline membrane begins, so that it occupies 

 a position between the first polar cell and the egg (Plate 11, Fig. 95, 

 mh.vL). This is followed by the development of a second polar cell 

 (Plate 2, Fig. 17), which lies within the vitelline membrane (Plate 11, 

 Fig. 95, cl.pol}). From the time of assuming the spherical shape, soon 

 after leaving the oviduct, the eggs retain this form, except when pressure 

 of surrounding eggs in the egg-lamellte distorts them. The egg repre- 

 sented in Figure 17 is an example of the influence of pressure in the egg- 

 lamcllse ; such a form at this stage has not been seen among eggs kept 

 isolated in watch glasses. It should be noted here that the uniform 

 distribution of yolk serves to distinguish such eggs, which are pressed 

 into an elongated shape, from later stages in which the eggs are normally 

 ellipsoidal even when isolated, but in which the yolk is collected at the 

 vegetative pole. 



Eggs which are isolated soon after oviposition retain the spherical con- 

 dition and the uniform distribution of the yolk until about the time when 

 the second polar cell is formed. Then the egg begins to elongate in the 

 direction of the chief axis, and the protoplasmic materials begin to con- 

 centrate at the animal pole, where the polar cells are located ; at the 

 same time the yolk is removed to the lower half of the egg, being con- 

 centrated around the vegetative pole. This movement of protoplasm 

 and yolk, towards animal and vegetative poles respectively, continues 



