946 



SCIENCE 



[N. S. Vol. XXVII. No. 703 



the factors determining the distribution of 

 the one species within caves, while the 

 nearly related form, though living in the 

 same region, did not take to that habitat. 

 Both species were found to be negatively 

 phototropic ordinarily, and were negative 

 to all intensities to which they responded 

 at all; but Asellus after having been in 

 darkness three hours or longer was positive 

 in its response to any intensity to which it 

 responded at all (2.2 candle-meters or 

 greater intensity). This affords an ex- 

 planation on the basis of reactions to light 

 alone of the occurrence of Ccecidotea within 

 caves and Asellus outside of caves. Caci- 

 dotea responds to such intensities as to 

 enable it to avoid daylight, while Asellus, 

 after once having been in darkness for a 

 time, is positive in its response to light, so 

 that, while the former would tend to re- 

 main within the darkness of eaves, the lat- 

 ter, if it got into a cave, would after a time 

 become positive in its response to light and 

 escape if it happened to come within reach 

 of light from the outside. Another pos- 

 sible factor in determining the cave dis- 

 tribution of the one and not of the other 

 species is the apparently greater discrim- 

 inative power of the Ccecidotea in selecting 

 food. Asellus takes with its food a large 

 amount of inorganic matter, while Cceci- 

 dotea, living as it does where organic mat- 

 ter is relatively very scarce, takes only a 

 small amount of inorganic matter with its 

 food. 



The Development of the Thymus in the 

 Pied-billed Grebe: Charles E. Johnson, 

 University of Minnesota. Presented by 

 H. P. Nachtrieb. 



The observations here presented are 

 based on a study of the thymus in Po- 

 dilymbus podiceps of approximately the 

 following ages: 4^, 5, 6, 6^, 7, 7|, 8 and 

 8| days. 



The embryonic thymus in this species of 



grebe is derived from two separate anlagen 

 the third gill-cleft and the fourth inner and 

 outer gill-pouches. The first anlage arises 

 as a pouch-like outgrowth of the third 

 gill-cleft and the second as a similar out- 

 growth of the fourth endodermal pouch, to 

 the lateral wall of which elements are 

 added from the fourth ectodermal pouch. 

 Through cell proliferation these pouch-like 

 structures become solid bodies, at the same 

 time becoming separated from the germ 

 layers and finally becoming united, on each 

 side, into a single body, the embryonic 

 thymus. The union or fusion into a single 

 body on each side appears to be largely due 

 to two causes. In the first place, the an- 

 lagen become straightened so as to come to 

 lie parallel with the long axis of the neck, 

 which tends to bring them together. In 

 the second place, the medianward shifting 

 of the jugular vein crowds the second an- 

 lage over against the first so that the an- 

 terior end of the second overlaps the pos- 

 terior end of the first. 



The thymus anlage furnished by the 

 third gill-cleft is larger than that arising 

 from the fourth giU-pouches, and the part 

 played in its formation by the ectoderm is 

 also greater in the former than in the 

 latter. 



Four well-defined ectodermal and endo- 

 dermal gill-pouches are present in embryos 

 about four and one half days old. There 

 is also present a postbranchial body, or 

 structure corresponding to a fifth endo- 

 dermal pocket, as an evagination of the 

 posterior wall of the fourth inner gill- 

 pouch where this opens into the pharynx. 

 Later on this postbranchial body becomes 

 constricted off and disappears rather rap- 

 idly. 



On the left side a portion of the third 

 gill-cleft lying between the pharynx and 

 the anlage of the thymus becomes converted 

 into an epithelial body. For a period of 

 about three days this body is continuous 



