Ch. V] EARLY DEVELOPMENT OF THE EMBRYO 57 



does take place, in the way shown by PA tiger's figures, 1 it can 

 only be due to a slight rotation of the egg as a whole in this 

 direction, for experiments show that the entire movement of 

 the ventral lip is forward, i.e. toward the dorsal lip. 



The yolk-plug is finally withdrawn into the interior of the egg 

 md the blastopore remains as a round or often somewhat elon- 

 gated opening. Its subsequent changes we shall follow later. 



External Changes after the Closure of the 

 Blastopore 



Let us next examine the changes that appear at this time in 

 the region that now lies anterior to the blastopore and on the 

 upper surface of the egg. There is much variation in the early 

 stages of development of the embryos of a given species, and 

 in different species the variations are even greater. The dif- 

 ferences in level of different regions are the result of move- 

 ments of the ectoderm. To see these to best advantage, the 

 living egg must be placed in the direct sunlight, and the surface 

 studied with low powers of the microscope. 



An embryo in which the yolk is still exposed is shown in 

 Fig. 20, A. Passing forward from the yolk-plug over the 

 upper surface of the egg is a broad groove, the so-called 

 "primitive groove." At the anterior end of the primitive 

 groove is a circular elevation. On each side of the primitive 

 groove, at IM, the inner medullary folds are seen. Outside 

 of these we find a depression, and farther on each side, at 

 EM, the outer medullary folds. A sickle-shaped depression 

 lies just in front of the blastopore. 



A later stage of the same embryo is shown in Fig. 20, B. 

 The primitive groove is narrower, the medullary folds are more 

 distinct, and anteriorly a continuation of the lateral folds has 

 formed. This will later be called the head-fold. Anteriorly 

 and laterally, there is formed on each side a lateral extension 

 of the medullary plate, the so-called "sense-plate." 



The medullary plates now begin to roll in, producing a deep 

 furrow, the medullary furrow with the primitive groove at its 





i Pfluger ('83), PI. II., Figs. 4 and 5 (see Fig. 18, F). 



