1892.] Experimental Embryology. 581 
especially as they furnish in themselves interesting facts bear- 
ing upon our interpretation of embryological phenomena. 
Without apparent cause, unless, as the author inclines to 
believe, old age of the adults is here concerned, all the ascid- 
ians obtained late in one season gave rise to abnormal eggs, 
few amid the entire number developing in the normal way for 
any length of time. These natural monsters or deficient eggs 
can be explained only on the assumption that the parent 
organism made them imperfect from the start, or at least fur- 
nished abnormal conditions of environment for them before 
they were laid, as they may develop in the same aquaria with 
other eggs that follow a typical series of changes without any 
abnormalities. Moreover the eggs from one adult often show 
some common defect or tendency to be abnormal in certain 
lines, though there is gront individual difference between even 
these eggs. 
These abnormalities of unknown or natural origin are class- 
ified under the following seven heads: Ist, change from the 
normal position of the cleavage planes; 2d, retarded cleavage ; 
3d, cleavage confined to the nuclei; 4th, absence of cleavage ; 
5th, fusion of cells; 6th, unusual migrations of cells; 7th, 
death of cells. The presence of one or more of the above fac- 
tors and their various combinations gives rise to the numerous 
monstrosities found during the cleavage, gastrulation and 
larval life; moreover one abnormality gives origin to others 
later on in development, so that larve with great defects are 
classed as cases of death, for instance, of one or more cells in 
an early stage. 
In addition to the various abnormalties thus classified some 
other forms, such as a larva with well-developed double or bifid 
tail, were observed but not traced back to any of the above 
seven categories. 
The interest of these various modes of irregularity lies, for 
our present purposes, in the fact that all seven conditions have 
been artificially brought about by M. Chabry by various 
mechanical stimuli applied either to the egg of the ascidian 
or to that of the sea urchin. The results upon the ascidian 
were for the most part obtained by means of traumatic inter- 
