524 INVEETEBKATA CHAP. 



affirmative. Morgan was the first to discover that the eggs of 

 Echiuoidea, if treated with hypertonic sea-water, that is, water iii 

 which the normal proportion of chlorides is increased, will occasion- 

 ally undergo irregular segmentation and develop into larvae with 

 feebly developed arms, which lie on the bottom. 



Loeb (1910) confirmed this result, but he showed that if the eggs 

 are first treated with an organic acid, like butyric or valerianic acid, 

 for a minute or two, they will form a membrane exactly like the 

 vitelline membrane ; then regular segmentation will commence 

 and lead to the formation of normal larvae which swim at the 

 surface. This occurs in but a very small percentage of cases, while 

 the rest perish by a process of cytolysis whereby the blastomeres 

 break up into globules. If the eggs, after being treated with the 

 organic acid, are rinsed in clean sea-water and then transferred to 

 hypertonic solution, and after a time to clean sea-water, all will 

 develop into normal larvae. It is to be noted that the time differs 

 in the case of the eggs of every female examined, and has to be 

 experimentally determined in each case. 



From such experiments Loeb draws the conclusion that the 

 formation of a vitelline membrane is an important step in normal 

 development : that this membrane is due to the beginning of a 

 process of cytolysis initiated by the spermatozoon, since careful 

 examination shows that the first stage in its development is the 

 formation of a layer of minute globules which flow together; but 

 that the spermatozoon also contains a material which checks this 

 cytolysis when it has gone far enough, and that both these actions 

 are independent of what may be called the hereditary influence 

 of the spermatozoon. 



2. We now pass to experiments made on embryos and larvae. 

 Driesch began his investigations (1892) by taking normally 

 fertilized eggs of sea-urchin, waiting until they had divided into 

 two or four blastomeres, and then separating them by violent shak- 

 ing. In this way he was able to show that even one cell of the 

 S-blastomere stage will produce a perfect larva, so that in this 

 species the division into cells is not, as Eoux supposed, a division 

 of the germ into areas destined to form special organs. 



He further showed that when one of the first two blastomeres 

 was separated from the other, the isolated blastomere segmented as if 

 it still formed half of the egg. Thus a hemispherical cup of cells was 

 formed, and the edges of this cup were then drawn together by 

 movements of the cells composing it. In this way a blastula was 

 formed with half the normal number of cells, which nevertheless 

 developed into a perfect larva. 



He was also able to induce the eight cells of the 8-cell stage 

 to arrange themselves in one plane, and this was accomplished by 

 freeing the eggs from their fertilization membrane by means of 

 shaking them a few minutes after fertilization, and by subjecting 

 them to pressure. If then the pressure were removed the eight cells 



