Apkil 4, 1902.] 



SCIENCE. 



525 



characteristic of swimming organisms 

 which follow a spiral path, keeping the 

 same side of the body always directed 

 toward the axis of the spiral. A large 

 number of organisms show this type of 

 structure. 



On the Early Development of Spermo- 

 philus Tridecemlineatus, a new Type of 

 Mammalian Placentation : Thomas G. 

 Lee. 



SpermopMlus differs from other rodents 

 in a temporally fixation of the blastocyst to 

 the antimesometrial wall of the uterine cav- 

 ity. Later the blastocyst detaches and the 

 true placenta develops on the mesometrial 

 wall. The uterine lumen resembles a capi- 

 tal T, the cross-bar being the mesometrial 

 side. Tubular glands open on all mucous 

 surfaces, later disappearing in the anti- 

 mesometrial region. The ovum, entering 

 the uterine cavity at the close of segmen- 

 tation, forms a small blastocyst consisting 

 of an outer or trophoblast layer and an 

 inner cell-mass, which differentiates into 

 ectodermal and entodermal portions. At 

 the antiembryonal pole of the trophoblast 

 a multinucleated syncytial mass develops 

 which projects from the free surface. This 

 fixation mass perforates the uterine epi- 

 thelium and touches the growing vascular 

 connective tissue. Enlarging, it becornes a 

 rounded mushroom-shaped mass, convex 

 next the connective tissue, cupped next the 

 blastocyst. The thin margins gradually 

 extend between the epithelium and connect- 

 ive tissue. Later numerous root-like pro- 

 cesses develop composed of fine longitu- 

 dinally striated protoplasm; these extend 

 into the connective tissue of the mucosa. 

 The anti-mesometrial portion of the uterine 

 cavity loses its epithelium and rapidly 

 dilates to accommodate the growing blas- 

 tocyst. The fixation mass becomes a more 

 and more shallow cup and the roots atrophy 

 and disappear; the result being the separa- 



tion of the blastocyst from its attachment. 

 By means of a zone of trophoblast external 

 to the germinal area the embryonal pole of 

 the blastocyst becomes attached to the mar- 

 gins of the transverse mesometrial portion 

 of the uterine cavity which retains its. 

 epithelial lining and forms the site of the 

 true placenta. Later development is sim- 

 ilar to that of the European form, Spermo- 

 pMlus citellus, described by A. Pleisch- 

 mann. • A detailed description of these 

 stages with plates and discussion of liter- 

 ature will soon be published. 



Demonstration of the Placentation of Sper- 



mophilus (stereopticon) : Thomas G. 



Lee. 

 Variation in the Box Elder Bug (Lep- 



tocoris) : H. B. Wakd. 

 Some Alaskan Sipunculids: H. B. Waed. 

 Cell-Homology: Edmund B. Wilson. 



In an analysis of the conception of cell- 

 homology, it was pointed out that here, as 

 elsewhere, the essential criterion of genetic 

 homology is that of common ancestral 

 descent, and that no purely embryological 

 criterion is in itself adequate. That cell- 

 homologies may be merely incidental or 

 secondary to regional homologies of the egg 

 as a whole applies equally to all forms of 

 genetic homology and constitutes no valid 

 argument against cell-homology ; but, owing 

 to the plasticity of cleavage-forms, cell- 

 homologies may be more readily modified 

 or even obliterated than other forms of 

 homology. For practical purposes cells of 

 like prospective value, giving rise to homol- 

 ogous structures, may, irrespective of their 

 origin, be called equivalent : those of like 

 ontogenetic origin and position may, irre- 

 spective of their fate, be called honiohlastic ; 

 but neither equivalent nor homoblastic cells 

 are necessarily homologous. The term 

 homology (partial or complete) is applica- 

 ble in cleavages of like pattern which have 

 been derived from a common ancestral 



