276 



These bands lie approximately parallel to the meridians of the egg 

 and converge at a point somewhat above the lower pole of the egg. 

 The extension of the spots into bands is due to the division and sub- 

 sequent migration of part of the daughter cells, of the equatorial cells. 

 The stained yolk granules furnish a record of the path that has been 

 taken by the moving cells. 



The mode of blastopore closure in Spelerpes is so unusual that 

 it seemed desirable that these experiments be extended to the eggs 

 of some amphibian with the usual mode of blastopore closure. In 

 Spelerpes the blastopore begins like that of the frog, but after the 

 lateral lips have extended so as to cover half of a circumference, 

 further progress ceases. Closure of the blastopore takes place by 

 gradual contraction of the blastopore into a straight line, which shortens 

 until at last only a minute median portion remains as the anus. The 

 blastopore, therefore, never becomes circular as in most other amphi- 

 bians, nor is there a definitive ventral lip. There is nothing unusual 

 about the formation of the embryo. 



During the past spring (1910), I repeated these experiments, using 

 the eggs of one or more species of Amblystoma and the eggs of one 

 species of frog. I did not succeed in obtaining the adults of the latter 

 so that the species is unknown. An attempt was also made to use 

 the eggs of the wood frog, Rana sylvatica, but the pigmentation in 

 these eggs is so intense that it renders them unsuitable for this work. 



When the eggs of Amblystoma are first deposited, the upper hemi- 

 sphere is brown, the lower cream color or sometimes light gray. The 

 separation of the colors is usually quite sharp, the boundary line ordi- 

 narily coinciding with the equator. The brown gradually spreads over 

 the lower hemisphere completing the process simultaneously with the 

 withdrawal of the yolk plug. The general color of the egg is now 

 lighter than the original. Often there are numerous dark stripes ex- 

 tending from the equator along the meridians of the egg to the blasto- 

 pore. Except for these stripes, the egg is now nearly uniform in color 

 throughout. When the embryo begins to form, the neural plate region 

 begins to darken, due doubtless to an increase in the amount of pig- 

 ment per unit of area in this region, which in turn is brought about 

 by the movement of the cells of the neural plate toward the midline. 

 Accompanying this movement is an extension of the cells of the ventral 

 ectoderm in order to occupy the region left by the inrolling cells of 

 the neural plate. As a result, the amount of pigment per unit of area 

 of the ventral ectoderm is decreased so that this region becomes re- 

 latively lighter than the dorsal region. 



