52 THE FROG 



Exercise 54. — The Late Gastrula. 



(b) Examine a late gastrula in which the cells of the germ ring 

 have encroached still farther upon the exposed area of the yolk-laden 

 cells. This gastrula is often called the yolk-plug stage because the 

 yolk-laden cells appear as a small plug on the surface of an otherwise 

 pigmented embryo. The crescent-shaped blastopore of the early gas- 

 trula has now become ring-shaped and has lateral and ventral lips. 

 Cell movements continue until the yolk-laden cells are overgrown com- 

 pletely and the blastopore is a minute opening. Draw the late gastrula 

 as seen from the vegetal hemisphere. 



(c) Examine a demonstration stained section which shows the 

 archenteron, or gastrula cavity, opening by way of the blastopore and 

 the blastula cavity which is being obliterated. With your handlens 

 try to determine the deepest region of the blastopore, that is, try to 

 locate the dorsal lip of the blastopore on one of your specimens. Cut 

 the gastrula into halves in a plane passing at right angles to the dorsal 

 lip and examine the cut surfaces under water, using the low-power 

 objective and reflected light. Good halves will show the cavities as 

 clearly as in the demonstration specimen and, in addition, the third 

 dimension can be seen. The cell movements of gastrulation which 

 give rise to the gastrula cavity and tend to obliterate the blastula 

 cavity bring about the first conspicuous localization of cells during 

 development, namely, the formation of the ectoderm, an external layer 

 of cells, and the endoderm, a layer of cells lining the archenteron. 

 These layers are known as germ layers and are continuous with one 

 another at the lips of the blastopore ; they arise simultaneously during 

 gastrulation. Since it is known from later development that the blas- 

 topore is at the posterior end of the embryo and the archenteron 

 nearer the dorsal than the ventral surface, it is possible now to begin 

 the use of the terms anterior and posterior, dorsal and ventral, as used 

 for the adult frog. Draw the cut surface of a half gastrula, oriented 

 in such a way as to place the future dorsal region toward the top of 

 the page. The future anterior end may be to the right or to the left, 

 but in subsequent figures the orientation here chosen must be con- 

 tinued. 



F. Formation of the Organ-System Primordia 



Exercise 55. — ^The Early Neural-fold Stage. 



(a) During the latter part of the localization of the ectoderm and 

 endoderm certain cells pass into the region between these two layers. 

 These cells make up the third germ layer, or mesoderm. At the same 

 time, a rod of compact cells is separated from all three germ layers in 



