TRANSACTlOiNS OK SECTION 1). bifL 



tained within a crypt in the distal wall of the uterine cavity, and the uterine 

 epithelium is disappearin^r from the walls of the crypt. 



2. A few hours later the primitive epiblast divides into formative epiblast and 

 trophoblast. 



3. During the latter part of the seventh day the trophoblast rapidly increases, 

 becomes closely attached to the decidua, and pushes the formative epiblast towards 

 the yolk sac, which becomes invaginated. The non-invaginated portions of the 

 yolk sac lie in direct contact with the decidua, in which numerous slit-shaped blood 

 spaces have appeared. 



4. In the early part of the eighth day the walls of the ovular crypt, which 

 sprang from the distal side of the uterine cavity, fuse with the proximal wall of 

 that canal, and thus the crypt is converted into a closed space, and the continuity 

 of the uterine canal is interrupted. The greater part of this space is occupied by 

 the ovum, but at the mesometrial and anti-mesometrial ends portions of the cavity 

 remain, and are transformed into maternal blood sinuses. The hlood in the meso- 

 metrial sinus bathes the proximal end of the trophoblast, and that in the anti- 

 mesometrial sinus bathes the distal end of the yolk sac. Further, by the 

 disappearance of the inner wall of the slit-shaped spaces at the sides of the yolk 

 sac the maternal blood is brought into direct relation with a large part of the 

 circumference of the yolk sac, and spaces which have in the meantime appeared in 

 the trophoblast also become filled with the same fluid. 



5. During the ninth day the coelom is formed, and the allantois, which is a solid 

 mass of mesoblast containing no diverticulum from the alimentary canal, grows 

 into the coelom, but it does not become attached to the trophoblast until the 

 eleventh day. 



6. Between the ninth and the seventeenth days the decidua reflexa gradually 

 separates from the distal wall of the uterus, and the continuity of the uterine canal 

 is re-established. The decidua reflexa is reduced to a thin membrane, and the 

 circulation within it ceases. When these changes are accomplished the distal part 

 of the vitelline cavity is obliterated by the apposition of its walls, but the proximal 

 portion remains ; and by means of diverticula, which project from it into the 

 placenta, the intimate relation of the yolk sac to the maternal blood is maintained 

 after the circulation in the decidua reflexa has terminated. 



7. The close relation of the yolk sac to the maternal blood suggests the idea 

 that the sac itself is an important agent in the early nutrition of the embryo, and 

 the peculiar relationship of the hypoblast to the placenta indicates the possibility 

 that the hypoblast cells play some special part in embryonic nutrition. 



5. Ohservations upon the Development of the Spinal Cord in Mus musculus 

 and Mus decumanus : the Formation of the tSepta and the Fissures. 

 By Arthur Robinson, M.B. 



1. At the eleventh day the spinal cord is a hollow rod of nucleated protoplasm. 



2. Within a few hours the neuroblasts are differentiated. 



3. On the twelfth day the formation of the grey matter commences, and tha 

 rudiments of the white columns appear. 



4. The antero-lateral white columns consist of nerve fibrils derived from the 

 neuroblasts of the cord embedded in a spougioblastic reticulum. 



5. The posterior white columns are formed by the processes of the neuroblasts 

 of the spinal ganglia. 



6. The spongioblasts of the dorsal and ventral walls of the centred canal are 

 drawn out into two septa, an anterior and a posterior, which extend respectively to 

 the ventral and dorsal surfaces of the cord. 



7. The extension of the anterior septum is due to the formation of the anterior 

 commissures and the shrinking of the central canal. 



8. The extension of the posterior septum is due principally to the formation of 

 the posterior columns, but also to the formation of the posterior commissure and 

 the shrinking of the central canal. 



T T 2 



