cyo MORGAN AND HAZEN. [Vol. XVI. 



corrosive-acetic solution the embryos stain well in picro-lithium 

 carmine. Without using the highest powers of the microscope, 

 very little can be made out of the cell-structure of the embryo. 

 Almost all our work, therefore, has been done with a Zeiss 

 immersion 2 mm. 



Gastrulation. 



One of the great difficulties in following the changes that 

 take place during gastrulation is due to the absence of land- 

 marks. Our attention has been largely directed toward the dis- 

 covery of points of orientation of the early gastrula. Hatschek 

 relied mainly on the form of the embryo in optical section, and 

 we have also found this, under certain circumstances, a valuable 

 means of orientation. Wilson noticed that the pore at the 

 vegetative pole, which is sometimes left at the end of cleavage, 

 persists occasionally throughout the early period of invagina- 

 tion, and by this means he showed that the vegetative pole was 

 brought into contact with the animal pole ; in other words, 

 that the invagination was radially symmetrical. We hoped at 

 first, by using this opening as a fixed point, to determine the 

 later changes that take place, but all traces of the pore soon 

 vanish, except in very abnormal cases. 



It is of importance to determine at as early a stage as 

 possible the orientation of the gastrula ; and here we have been 

 more successful, since we have been able to distinguish the 

 dorsal and ventral lips of the blastopore at the beginning of 

 the gastrulation process. The gastrula is not perfectly sym- 

 metrical, and the same is probably true of the blastula, 

 although more difficult to demonstrate. During invagination 

 the vegetative pole is brought near the animal pole, yet the 

 endoderm turns in such a way that the dorsal side can be 

 distinguished from the ventral. A careful study of the yolk 

 granules, and their appearance in the cells, has aided greatly in 

 orientation. Certain cells, that are turned in at one side, contain 

 relatively fewer and lighter yolk granules, and these cells can 

 be traced from the first stages of invagination until the closing of 

 the blastopore. They mark the dorsal side. As this is a con- 

 stant feature, it gives a definite means of orienting the embryo. 



