DEVELOPMENT OF SPELERPES BILINEATUS 239 



expect that an egg forced to form a spina bifida embryo would 

 form more than a half structure on one side. In these spina 

 bifida embryos, it is interesting to note that the material differ- 

 entiates a little differently than in the normal embryo. That is, 

 material which, in a normal egg, might form part of the tail, in 

 these embryos, differentiates into the parts in their consecutive 

 order from head to tail. This is to be expected, since Morgan 

 ('02) has already pointed out that ring embryos may be derived 

 from material which ordinarily would not form the embryo. 

 Since the urodelian embryo forms in greater proportion above 

 the equator than does that of most Anura, it follows that the 

 material of the dorsal lip in the former is less complex than that 

 of the latter. The normal dorsal lip of the frog contributes 

 material to all parts of the embryo from the neck to the tail. 

 That of Spelerpes contributes material to onl}^ a small part of 

 the posterior end of the embryo. 



SUMMARY 



1. The uncleaved egg shows three zones of yolk granules, 

 an upper of small ones, a second of larger size, while the lowest 

 contains the largest granules. The anterior part of the embryo 

 develops from the small granules. From the second zone come 

 the posterior part of the embryo in the narrow sense and the pos- 

 terior part of the archenteric roof besides the ventral ectoderm. 

 The zone of large granules furnish the rest of the emVjryo. 



2. Cleavage is total, unequal and often irregular. After the 

 fourth division, it is confined to the upper hemisphere for a long 

 time. 



3. Sometimes the third set of furrows change during their 

 formation from a horizontal to a vertical plane. 



4. Sections of eggs in cleavage stages show first, a cap of small 

 cells, then a layer of larger yolk cells and beneath this, forming 

 the greater part of the egg, is the yolk mass, scarcely divided at 

 all into blastomeres. 



5. The blastomeres often remain connected for a long time. 



6. The egg increases in size up to the time of gastrulation. 

 After the embryo begins to appear, it decreases slightly. 



