May 17, 1907] 



SCIENCE 



111 



halves. Then later from the posterior 

 end of the anterior half on each side 

 a vesicle grovi's out. These vesicles are 

 the left and right hydrocoeles. The latter 

 though rudimentary in the adult is at 

 first just as large as the left one which gives 

 rise to the water-vascular system. The 

 right hydroccele in Astei-oidea and Eehi- 

 noidea has from the first a position near 

 the mid dorsal line, on which account some 

 have doubted its homology with the left 

 hydroccele. But there can be no doubt of 

 its homology in Ophiothrix fragilis where 

 it is not only normal in position but some- 

 times assumes a five-lobed form similar to 

 that of the left hydroccele. Its dorsal posi- 

 tion near the primary water-pore in Aster- 

 oidea and Echinoidea is accounted for by 

 the fact that in these groups it is oriented 

 not with respect to the larval mouth but 

 with respect to the permanent mouth which 

 is on the left of the larva. In Ophiuroidea, 

 where the larval mouth persists as the 

 adult mouth, it is accordingly found un- 

 mistakably on the right side. Subse- 

 quently the preponderant growth of the 

 left hydroccele and all the structures asso- 

 ciated with it carries the water pore dorsal 

 to the mouth over to the right side where it 

 comes to lie near the right hydroccele, a 

 position which it occupies from the begin- 

 ning in Asteroidea and Echinoidea. 



Exhibition of Embryos of the Japanese 

 Frilled Shark, Chlamydoselachus angu- 

 nieus, with Comments upon its Plan of 

 Development: Bashford Dean, Colum- 

 bia University. 



Mercator Projections of Vertebrate and 

 Arthropod Embryos: "William Patten, 

 Dartmouth College. 



The Pre-placental Development in Geomys 

 bursariiis: Thomas G. Lee, University 

 of Minnesota. 

 This investigation is a continuation of 



the writer's comparative studies upon the 



earliest stages of development in North 

 American Eodentia. 



Geomys bursarius, or the 'pocket gopher' 

 as it is commonly called, is characterized 

 by the so-called inversion of the layers 

 which is of a much simpler type than that 

 found in other rodents, as the mouse and 

 guinea-pig. 



Development takes place entirely outside 

 of the uterine cavity in a decidual cavity 

 formed by the vascularization and break- 

 ing down of the ventral uterine connective 

 tissue. The ^idennic blastocyst perforates 

 the epithelii, n lining the ventral portion 

 of the uterin^ cavity. This perforation is 

 relatively la.r^e and does not become closed 

 as in the guinea-pig or in man, but remains 

 open for a considerable period. The 

 epithelial lip of this rounded opening be- 

 comes somewhat thickened and everted. 



The trophoblast, in a zone a little way 

 external to the germinal area, becomes ad- 

 herent to the outer margins of this epithe- 

 lial lip, thus suspending the blastocyst 

 while the decidual cavity is being hollowed 

 out beneath it in the connective tissue. 



Rauber's layer disappears from the sur- 

 face of the germinal area at about the 

 time of perforation. The entoderm forms 

 a well-marked vesicle or yolk-sac which 

 rapidly increases in size and which be- 

 comes invaginated on its dorsal surface by 

 the sinking in of the germinal area, thus 

 bringing about the so-called inversion of 

 layers. 



The amnion is formed by the folding 

 over and fusion of the outer margins of 

 the germinal area from all sides, while the 

 outer portion of these folds, composed of 

 trophoblast, form a membrane constituting 

 the serosa or false amnion. This serosa 

 now closes the opening through the uterine 

 epithelium above referred to. 



At a later period, secondary folds of the 

 serosa appear which unite forming a two- 

 layered cup or vesicle of trophoblastic 



