March 27, 1903.] 



SCIENCE. 



493 



istics of the Tornaria larva. Field was 

 not able to detect any fibrils associated 

 with the thiekening in the larva3 he ex- 

 amined, but in the larva of Echinus 

 esculentus the layer of fibrils above de- 

 scribed goes on increasing in thickness as 

 growth proceeds, until just before the 

 metamorphosis it is as thick as the cells 

 themselves, and intermixed with the fibrils 

 are a few minute ganglion cells of the type 

 commonly found in the nervous system of 

 echinoderma. The discovery of this brain 

 removes a great difficulty in the way of 

 comparing the larviB of echinoderma with 

 the Tornana larva. 



The Effect of Lithium Chlonde on the De- 

 velopment of the Frog's Egg: T. H. 

 Morgan, Bryn Mawr College. 

 In 1894 I tried the effect of several solu- 

 tions on the development of the frog's egg; 

 amongst others, solutions of several halogen 

 salts. The main result was to produce 

 spina bifida embryos. A year later Hert- 

 wig extended the same experiment, and in 

 1896 Gurwitsch also described the effect 

 of a number of substances, including lith- 

 ium chloride, on the development of the 

 frog's egg. The interpretation given by 

 Gurwitsch of the kind of embryos pro- 

 duced by solutions of this salt did not ap- 

 pear to me to fit in with results that I had 

 obtained in other ways. This led me to 

 take up the subject again. Amongst the 

 different kinds of embryos that I obtained 

 there were some similar' to those described 

 by Gurwitsch. I shall not, however, de- 

 scribe here embryos of this sort, nor dis- 

 cuss their interpretation.* Amongst the 

 embryos there was a characteristic kind, 

 different from any that have been yet ob- 

 tained. It is these that I shall now de- 

 scribe. 



* Madame Kondeau-Luzeau has more recently 

 (1902) described the eflfeot of lithium chloride on 

 the frog's egg. 



Eggs in the two- and four-cell stages, as 

 well as in early and later segmentation 

 stages, were put into fresh water to which 

 0.4, 0.5, 0.6 per cent, lithium chloride had 

 been added. The best results were obtained 

 from eggs in the late segmentation stages. 



There appears after several days in the 

 eggs in the solutions an invagination on 

 one side of the egg. A little later a cres- 

 centic depression or even a complete ring 

 appears high up on the egg, and the whole 

 black hemisphei'e seems to be sinking into 

 the interior of the egg, with the crescent 

 or ring closing over the top of the egg. At 

 the same time a slate-colored band appears 

 in the region between the first invagination 

 and the ring above. This band is much 

 broader on that side of the egg where the 

 invagination first appeared. Along the 

 middle of this area a darker line runs 

 vertically upward. I may say at once 

 that this line indicates the position of the 

 notochord, and the slate-colored band is a 

 layer of endoderm cells, one cell deep. Be- 

 neath it are the two mesoblastic sheets, 

 one on each side of the notochord. 



Sections of these eggs show clearly what 

 has taken place. The top of the egg that 

 disappeared into the interior forms inside^ 

 of the egg the medullary plate, bent double 

 on itself. It lies, therefore, in the middle 

 of the egg. As the whole ectoderm has 

 turned in, the yolk-cells from the sides 

 have been drawn upward, where they form 

 thfe single layer of cells that cover the slate- 

 colored area. Beneath this lies on each 

 side of the notochord a thick mesoblastic 

 sheet. 



The first invagination (archenteron) 

 sinks deep into the egg— possibly it is con- 

 tinued by the yolk cells drawing apart. 

 A narrow 'archenteron is formed in this 

 way, that bends uader the medullary plate 

 in the interior of the egg. The notochord, 



