STRUCTURE AND DEVELOPMENTAL RATE 153 



exhibit almost any known defect. The fins may be poorly 

 developed and the bodies ill-shaped and twisted. The tails may 

 be short, bifed, and undeveloped due to a slow or arrested descent 

 of the germ-ring. And finally there may be such minor defects 

 as would escape observation until the hatched embryos were 

 found to be unable to right themselves and swim. These are the 

 defects to be seen on simple external examination, the internal 

 structures are as frequently abnormal. The latter fact is borne 

 out by numerous examinations of these monsters in sections. 

 I have studied a great many of the sectioned specimens during 

 the past number of years. 



The reason for this great variety of monsters following arrests 

 during cleavage stages is that the development of all organs 

 or parts must subsequently take place and all may thus become 

 arrested and deformed. When eggs are treated at later stages, 

 as at the beginning of gastrulation, no double monsters will 

 occur, their moment has passed, though the various brain, 

 branchial, and other defects mentioned may exist. When treated 

 after the embryonic axis is visible, it is most difficult to get any 

 gross eye defects and so on. 



Thus it may be said that the earlier the arrest the more numer- 

 ous will be the type of defects found and the later the arrest the 

 more limited the variety of deformities, since there are fewer 

 organs to be affected during their rapidly proliferating primary 

 stages. 



The same treatment that causes a gross deformity when ap- 

 plied during an early stage, will during a later embryonic stage 

 often give only a minor effect. 



The further records of experiments will render these statements 

 more fully certain. Here I wish simply to call attention to the 

 great variety of gross deformities resulting from these early 

 arrests. The contrasts in detail between these and the later 

 treatments will be shown in the following pages. 



I hasten, however, to caution any experimenter who may in 

 the future find a double monster or cyclopean monster, for ex- 

 ample, in a group of eggs arrested or treated during late develop- 

 mental stages, not to assume that this is due to the late treat- 

 ment or that it disproves the standpoint stated above. For 



