ON THE DEVELOPMENT OF THE FCETL'S. 9 



la proof of the correctness of this opinion, we may refer to the experiments performed by 

 M. Gaspard, (Jourtt. de Physiul. Experiment, No. 3,) in order to ascertain this point, and 

 to the formation of blood in the impregnated egg on the second or third day after incuba- 

 tion, and to the fact that, in the numerous tribe of oviparous animals, the foetuses are insu- 

 lated from the mother, and are the real manufacturers of their own blood. 



At an early period of fcctal life the ganglial ramifications and centres are first formed, 

 and afterwards the ramifications and centres of the voluntary nerves. In the more perfect 

 animals, even that part of the nervous system which is general throughout the animal crea- 

 tion, and which the lowest orders of it possess, is the first formed, and that part which is 

 destined to perform the highest functions, and which the perfect animals only possess, is 

 produced the last. 



Other peculiarities of the fcetus are adduced in the subjoined notes: 



Of the Development of the Heart and Ltaigs. J. F. Meckel has concluded from his ob- 

 servations, 1st, The heart is relatively larger the younger the embryo. For in his observa- 

 tions he found, at the first period, at which the heart could be distinguished, that it filled 

 completely the thoracic cavity. 



2d, The heart is more symmetrical with respect to situation and form, soon after its for- 

 mation, than at a more remote period. 



3d, The form of the heart undergoes various changes during the growth of the ftetus. 



Jl. The proportion between the arterial and venous portions of the heart is not always 

 the same. The auricles surpass the ventricles in capacity, in proportion as the embryo is 

 younger. 



.B. The relative volume of the two sides of the heart is not always the same at all pe- 

 riods. In the adult the right side always more or less exceeds the left; but in very young 

 embryos the two ventricles are equally capacious, but that of the right side increases ra- 

 pidly. The right auricle surpasses the left in size in the foetus, and it is only by degrees 

 that the left becomes equal to the right. 



C. The right ventricle is unquestionably smaller than the left at first. 



D. The thickness of the parietes of the heart is much more considerable at first. The 

 two halves of the heart equally present this difference, but the right ventricle always ap- 

 pears a little thicker than the left. This is, however less, the younger the foetus. 



-B. The two ventricles communicate with each other at an early period, and, according 

 to all appearances, continue to do so until the end of the second month, by means of an 

 opening in their interior aspects, situated at their base, and immediately beneath the origin 

 of the great vessels. 



F. The interior disposition of the auricles with respect to their communication, either 

 with one another, or with the venous trunks, undergoes considerable changes. These turn 

 chiefly on the form and size of the oval hole, the situation of the orifice of the vena cava 

 inferior, the situation, the form, the extent and relations of the valve of Eustachius and 

 thit of the foramen ovale. Here M. Meckel's researches confirm those of Sabatier and 

 Wolff. 



1th, The disposition of the aorta and of the pulmonary artery offers several considerable 

 changes, in succession, of which the following are the chief. 



A. At first there exists only an aorta. A pulmonary artery is formed at a remoter pe- 

 riod. It is not until after the seventh week that the pulmonary artery begins to appear, 

 and then it is only a second aortic trunk, as yet without branches a right aorta, proceeding 

 in the direction of the lungs, which are very distant, and extremely small. 



The disposition of tLe large arteries at this period (seventh week) nearly resembles what 

 it Continues to be, in reptiles, during the \vhole life of the animal. 



B. It is in the course of the eighth week only, in which the branches of the pulmonary 

 artery can be discovered. They are then much smaller, when compared to the trunk of 

 the artery and to the arterial canal, the younger the embryo. At five months they become 

 equal to this canal, and afterwards they surpass it, frequently so far that when the fcetus 

 has completed the ninth month, each principal branch of the pulmonary artery is as large 

 as it is, or even larger. 



The venous canal presents similar appearances. It is during the first periods of the ex- 

 istence of the fetus when it offers, proportionally, the greatest amplitude. All the obser- 

 vations which M. Meckel has made, confirm this law, which is the more important as it 

 throws considerabie light on the functions of this canal. Indeed, it is probable that this 

 conformation is only the remains of a disposition which may be seen at the epoch when the 

 liver has not come into existence, when the vena porta and the vena cava inferior form but 

 one trunk, as the pulmonary artery forms, at the early stage of its existence, only one with 

 rta. This conjecture respecting the origin of the venous canal is confirmed by the or- 



