1893.] Embryology. 1101 
into three regions as it would be if it grew inside the ectodermal vesi- 
cle or body wall. 
The entodermal tract remains small—does not swell out as in the 
lithium larvee—and ultimately shrinks and falls off. 
In this way anenteria are formed, or plutei having no entoderm. 
Yet in these there is formed a small oral invagination without the 
presence of the entoderm. Such anenteria livéd a week, but did not 
regenerate the digestive tract. 
The second chapter deals with the effects produced by water con. 
taining less than the normal percentage of salts. Eggs of Echinus as 
soon as fertilized, are put into sea water to which fresh water has been 
added in the proportion of 45°5.; after five minutes they are transferred 
to 40°10 and then 35:15 and so on to 25°25 
The eggs swell, cleave normally in 45:5 but abnormally in 40:10 and 
35°15; while in 30°20 only the nuclei divide; in weaker solutions the - 
eggs die. The abnormal cleavage above mentioned consists in the 
unequal size of the cells formed in the eight cell stage. Here there are 
2 to 4 cells so small that they may be called micromeres. In the 
weaker solution, 35°15, there is added tbe shifting of the cells into a 
tetrahedral arrangement not otherwise present. 
Having thus produced apparent micromeres by varying the percent. 
age of salts in the sea water, the author regards the normal micromeres 
as not essentially different from the macromeres in their nuclei or idio- 
plastic parts, but as merely a result of the activity of the protoplasm; 
protoplasm may be effected by external agencies so as to make micro. 
meres when they are not predetermined by any character of the 
nuclei. 
The third section takes up his previous conclusions as to the inter- 
changeableness of the ectoderm and entoderm in their beginning. 
Not satisfied with the facts formerly relied upon to show that what 
would be ectoderm cells could form entoderm and vice versa, attempts 
were made to determine the relation between the position of the micro- 
meres and the place of origin of the entodermal invagination. But 
the experiment failed, since the eggs revolved in the capillary tubes in 
which they were reared and so made conclusions invalid. A second 
series of experiments was successful in showing the: dependen. of 
micromeres and entoderm and also in 
destined to form the vegetative side of a blastula could form a complete 
larvæ, while cells that should have formed the other half of the blas- 
tula could also form a complete larvæ. 
_ These experiments consisted in shaking off one or the other sets of 
