26 SUMMARY OF CUBRENT RESEARCHES RELATING TO 



motor organs ; these points he looks upon as having less morpho- 

 logical value than the development of the system, and its correlation 

 with other organs during the course of development. The closure 

 of the medullary groove is regarded as being merely the result of 

 further modifications. 



If we accept the general homology, we have next to determine 

 how the parts correspond ; the author cannot follow Dohrn and 

 Hatschek in regarding the homology as being complete ; he looks 

 upon the brain of Vertebrates as being a new formation, which is 

 their exclusive property ; it merely consists in an elongation and 

 dilatation of the already existing nervous system, or in other words 

 the medulla, which is the analogue of the ventral ganglionic chain of 

 the Articulata. 



The mesodermal derivates are dealt with in great detail, and a 

 comparison with what is seen in Plagiostomi leads the author to say 

 that they approach the higher, while the Ganoids approach the lower 

 Vertebrata ; and this portion of the essay concludes with an account 

 of the development of the enteric tract. 



Development of Petromyzon Planeri.* — J. P. Nuel directs 

 attention to the phenomena of the contractility of the ovum : imme- 

 diately after impregnation, before which the vitellus was everywhere 

 closely applied to the chorion, the yolk commences to contract, till 

 at last it is at all points separated from its investment. Calberla 

 regarded this as being due merely to osmotic action, but the fact 

 seems to be that a contractile wave, starting from the active pole, 

 slowly but gradually passes over the whole of the yolk ; this takes 

 about twelve minutes to be eifected. 



From the moment when the egg begins to segment there is 

 a period of rest between each division, and this period shortens 

 as development advances ; when the segmentation period is at 

 an end the cells of the liypoblast are in rejjose for a lengthened 

 period, while the epiblastic cells, continuing to divide, give rise to 

 an epibolic invagination. At a certain period most of the hypoblastic 

 cells start into activity, and the elements of the digestive tract begin 

 to be formed ; some of them, however, still remain quiet, and, only 

 later, give rise to the liver. When a group of cells enter into 

 activity, their calibre diminishes, and the yolk-grains are fused 

 together. 



After describing the details of the develoj^ment of the digestive 

 tract, M. Nuel states that the transformation of the yolk-spheres 

 first takes place along the axis of the embryo ; commencing at the 

 anus of Eusconi, it rajjidly extends forward ; being most intense at 

 the point where the ejiiboly is most advanced ; thence it widens out, 

 and gradually invades the whole surface of the hypoblast, till it 

 comes into contact with the segmentation cavity. 



When the mesoblast developes, it is clear that it has no relation 

 to the cliorda dorsalis ; for the two are simultaneously difi"erentiated 

 from a common embryonic layer, which, later on, also gives rise to 



* Arch, de Biol., ii. (1881) pp. 403-51 (2 pis.). 



