Prof. Retzius on the Form of the Cranium in the Embryo. 447 



same quantity of magnetic medium is displaced by the less magnetic 

 matter after, as there was before compression. Why, then, should 

 diamagnetic action be increased by compression ? 



Lastly, with respect to the crystals of carbonate of iron and car- 

 bonate of lime. Professor Williamson's explanation, although inge- 

 nious, is liable to the same objections as those already mentioned. 

 It cannot, in fact, be said that the functions of matter predomi- 

 nate most strongly over those of the medium they displace in any 

 one direction, merely because the particles maybe closer together in 

 that direction ; for, as long as each particle is surrounded by the 

 medium, the predominance of that function of the particles with 

 which we are concerned, i. e. their attraction, over that of the me- 

 dium they displace will be the same, whatever may be the distances 

 of the particles asunder. 



From the foregoing remarks, therefore, it is manifest that, if Pro- 

 fessor Tyndall has not yet succeeded in demonstrating that the 

 hypothesis of the existence of a magnetic medium and of the iden- 

 tity of magnetism and diamagnetism is necessarily at variance with 

 experimental facts, neither has Professor Williamson succeeded in 

 proving that this hypothesis is in accordance with those facts. The 

 question of the existence of a magnetic medium is still an open one, 

 and will continue to be so until the many important principles which 

 it involves, but which have not been introduced into the present 

 discussion, have been further elucidated by new investigations and 

 new thoughts. 



" Anatomical Notices." By Professor Andrew Retzius, of Stock- 

 holm. Extracted from a Letter to Dr. Sharpey, dated 10 May, 

 1855. (Translation.) 



" 1. On the Form of the Cranium in the Embryo. 



" So far as I am aware, due attention has not hitherto been given 

 to the different forms presented by the cranium in its earlier stages 

 of growth. In the skeletons of early human embryos to be seen in 

 most museums, the imperfectly ossified cranium is for the most part 

 shrunk up and disfigured. To obtain a correct view of the form of the 

 cranial cavity, I first remove the skin, fascia and muscles ; I then, by 

 injecting water through the vertebral canal, thoroughly wash out 

 the soft brain and spinal cord ; and lastly, fill the cerebro-spinal 

 cavity with quicksilver or with melted tallow, taking care not to 

 distend it over-much. The opening in the vertebral canal is to be 

 stopped with a little plug of wood, and the preparation allowed 

 to dry. 



" In the skeleton of a human embryo of the fourth month, pre- 

 pared in this way, the occipital bone was found to have the form of 

 a funnel, the narrow part of which passed into the vertebral canal, 

 as represented in the accompanying figure I. 



" It thus appears that the human occipital bone, in its early con- 

 dition, a|)proaclies in form to the vertebral canal, and in this respect 

 also it resernl)les the occipital bone in several quadrupeds, which so 

 obviously represents the first cephalic vertebra. 



