17 



aration aflFords a demonstration that young ova are single cells with 

 nucleus and cell-body. Sketch one or more that show typically. 



2. Ovarian Ova. Ambly stoma. Demonstration. Study, 

 recognizing in the young ova nucleus and cell-body. Compare 

 with (i). 



3. Early blastula stage. Ambly stoma. Assigned for study. 

 At this stage the fertilized ovum has divided up into a few cells, 

 arranged in a single layer around a central cavity (blastoccEle), — 

 comparable to a simple epithelium. Make a sketch showing out- 

 lines of cells and, if possible, of nuclei. 



4. Late blastula stage. Amblystoma. Assigned for study. 

 The cells at this stage have greatly increased in number, are smaller, 

 and the wall bounding the cavity is of several layers of cells, — com- 

 parable, therefore, with a stratified epithelium. Sketch, showing 

 the outline of the ^^%, the outline of the cavity, and for a portion of 

 the wall, the outlines of the cells and nuclei. 



5. Medullary plate stage. Amblystoma. Assigned for 

 study. At this stage differentiation is well advanced. The three 

 germ layers are seen in the section ; from the entoderm the noto- 

 chord is forming ; from the ectoderm, the central nervous system ; 

 the cavity is the alimentary canal (archenteron) . Sketch the dorsal 

 portion, and compare with the figure in the text-book. 



