2SO COMPARATIVE ANATOMY 



and we left the ovum as a sphere whose walls are composed 

 of several layers of cells enclosing an excentric cavity. One 

 half of the sphere is composed of small and deeply pigmented 

 cells, the other half is composed of much larger white cells filled 

 with granules of food-yolk. Thus we may speak of a pigmented 

 hemisphere and a white hemisphere, and it is important to 

 observe that the former always turns uppermost in whatever 

 position the egg may be placed. The cavity is the segmenta- 

 tion cavity or blastocoele. Close inspection shows that all the 

 cells of the pigmented hemisphere are not alike, but some of 

 them on one side are smaller and lighter in colour than the 

 others. When the pigmented cells reach down to the equator, 

 or a little below the equator of the egg, a short sti'eak or 

 furrow, rendered conspicuous by the deeply pigmented cells 

 bounding it, makes its appearance just below the equator of 

 the egg on the side occupied by the smaller cells. The groove 

 soon becomes a crescent with the concavity turned towards the 

 white hemisphere, and the Hmbs of the crescent gradually 

 extend along the lower sides of the white hemisphere. The 

 groove which becomes crescentic is the first indication of 

 the formation of the enteron, and will be referred to as the 

 dorsal lip of the blastopore. The changes which follow on 

 its appearance and accompany its extension require the' most 

 careful attention. The eggs, as has been said, float with the 

 pigmented side uppermost. In order to see what is going on 

 on the lower side one must place them on a strip of looking- 

 glass, cover them with a just sufficient depth of water and study 

 the reflections of the lower or white hemispheres.* The 

 external changes are represented in the series of drawings, fig. 

 58, A to J. In A we see the lower white hemisphere slightly 

 tilted towards the bottom of the page, so that part of the 

 pigmented hemisphere is seen at the top of the figure. The 

 small transverse groove forming the dorsal lip of the blastopore 

 is seen at dl., just below the equator of the egg. In B the 

 pigmented cells of the upper hemisphere have grown down- 

 wards over the cells of the white hemisphere at all points, 

 but most rapidly in the upper part of the picture. The dorsal 

 lip of the blastopore is now a crescentic groove, the horns of 

 the crescent forming the anterior boundary of the white area 



* I am indebted to Mr J. W. Jenkinson, M. A., for the following details 

 corroborative of PflUger's account of the closure of the blastopore. 



Digitized by Microsoft® 



