STRUCTURE AND DEVELOPMENTAL RATE 167 



mental period. Either stopping development or greatly reducing 

 its rate during cleavage stages and before the germ-ring has 

 formed, that is, at periods preceding gastrulation, frequently 

 serves to cause doubleness in the subsequent embryo formation. 

 Specimens subjected to any degree or kind of treatment after 

 the gastrular period never produced double or twin embryos. 



Subjecting Fundulus eggs to low temperatures during early 

 cleavages, the four-, eight-, or sixteen-cell stages, not only arrests 

 the cleavage process, but on later resuming development many 

 eggs fail to establish a normal rate and balance for some time and 

 the early processes of gastrulation would seem to be disturbed. 

 The majority of eggs after a stoppage of cleavage are completely 

 unable to resume development and may live for a few days in an 

 almost stationary condition and then die. Other arrested cleav- 

 age caps undergo a breaking-down or falling apart of the indi- 

 vidual cells before the death of the eggs. A small minority of 

 these hardy eggs after an arrest during cleavage stages succeed 

 in finally readjusting their development to a sufficient extent to 

 give rise to apparently normal free-swimming young fish. The 

 individual variations in resistance and developmental ability 

 shown among Fundulus eggs are remarkable in all experiments 

 performed on them. Our present consideration is to be centered 

 on that group which is sufficiently viable to continue develop- 

 ment, but not so resistant as to be able to completely readjust 

 its developmental processes following the early interruption. 



Not only does the entire experimental lot become divided into 

 the three above crude classes, but the members of our selected 

 group which is not completely capable of normal readjustment 

 by no means all develop in a similarly defective fashion. These 

 discrepancies again are due to individual variations in the man- 

 ner of resuming development. 



Certain specimens after removal from low temperatures 

 resume their cleavages with a fairly normal rhythm and form a 

 typical embryonic shield, but later the larger diverticula from 

 the interior parts of the central nervous system fail to arise in a 

 usual manner, or other processes requiring a high degree of devel- 

 opmental energy are not sufficiently expressed and various de- 



