38 METAMOEPHOSES OP MAN 



in their neighbourhood continue their development, 

 yet they remain in their primitive condition, and 

 may be detected in the adult, where they testify to a 

 former state of things very different from the existing 

 one. Thus we perceive that the history of embryonic 

 development may be summed up as consisting in 

 incessant transformations and constant activity.* 



This condition of change is nowhere so strongly 

 marked as in the circulatory system. Having for its 

 office to supply the organs of the body with nutritious 

 materials, it shares all their alterations. The various 

 branches and ramifications of the vessels increase with 

 the organs they supply with blood, and when the 

 latter disappear, the blood-vessels undergo a propor- 

 tionate diminution. Besides, this apparatus has its own 

 peculiar changes, which affect not only the central 

 portions and ultimate branches, but even the heart 

 itself. The latter makes its first appearance as a 

 transparent, solid, straight or slightly curved cylinder, 

 which is converted into a tube by resorption of the 



# In treating of the embryogenical changes of mammals, it is 

 impossible to rise above vague generalizations. I cannot even 

 mention the names of the authors to whom we are indebted for 

 many of these interesting facts. There are very many of them, 

 and besides, the memoirs and general works on this subject are 

 rapidly increasing in number. However, I may mention among 

 others, those of Messrs. Baer, Barry, Bischoff, Burdach, Coste, 

 Dumas, Duvernoy, Flourens, Hausmann, Henle, Huschke, Kolliker, 

 Lebert, Martin Saint- Ange, Meckel, Muller, Oken, Owen, Pur- 

 kinge, Eathke, Eeichert, Bemak, Schultze, Serres, Schwann, Thom- 

 son, Valentin, "Wagner, Weber, &c. M. Bischoff published a most 

 interesting and valuable work, combining his own researches and 

 those of other embryologists, which has since been translated into 

 French under the title of " Traite du Developpement de l'Homme 

 et des Mammiferes." Paris, 1843. 



