LECTURES ON EMBRYOLOGY. 



101 



power, is one of the most mysterious facts of phy- 

 siology, for which we have not yet any clue. 



The middle layer of the germ is that which pro- 

 duces the organ of circulation. First, the blood ap- 

 pears by a simple process of liquifaction of the 

 the cells. It can be seen under the microscope 

 how the particles, or the cells of that layer (Plate 

 VIII, fig. H) begin to be loose at the outer margin, 

 and to move between themselves, and to run in 

 particular directions, and to combine into currents, 

 and those currents to assume particular directions, 

 and then introduce a regular circulation before 

 there is a heart, and before there are blood-vessels. 

 It can be seen in every chicken under so low a 

 magnifying power that no one should lose the op- 

 portunity of seeing this wonderful sight. When 

 blood corpuscles move from the centre towards 

 the margin of the germ, the other cells which be- 

 come loose in the periphery of the germ (Plate 

 VIII, fig. I) begin to move towards the centre. In 

 the beginning, (Fig. H) there being no cur- 

 rent circulating, the two collections of fluid meet, 

 and finally (Plate VIII, fig. K) become regu- 

 lar currents by means of channels, through 

 which the blood runs for a regular circulation. 

 But there are constant changes in this current, and 

 even the ramifications of the blood-vessels are 

 constantly changing. One channel is left, and ano 

 ther is formed. It is like a river on the flat lands, 

 where the channel is constantly changing. And 

 here the channels of the blood are constantly dis- 

 appearing and are constantly reproduced. 



The lower layer forms, (Plate VIII, fig E) in the 

 more circumscribed parts of the body, a tube 

 which is to be the alimentary canal; and that con- 

 tracted portion of the tube opens into the yolk, or 

 a passage is formed, through which the alimentary 

 canal communicates with ths remaining yolK. 



It would lead me too far to notice the successive 

 changes which these organs undergo. I will only 

 show that we have some remarkable differences in 

 the successive transformations of the different 

 classes of vertebrated animals. In the Fish, (Plate 

 I, fig. H,) the same changes occur which we 

 have already noticed, but the final development 

 does not come up to what we have in the Bird. 



The germ, during the whole growth, is not sur- 

 rounded by special envelopes derived from its own 

 body (Plate I, fig. G) ; iris only the vitelline mem- 

 brane which surrounds it as it rises from the yolk 

 when the head and the tail is separated from the 

 yolk; the germ being never enclosed in any other 

 membrane. But now in Snakes, in Turtles, in Liz- 

 ards, in Birds, and in all Mammalia,there is an ad- 

 ditional envelope all around the embryo. And 

 this envelope is derived from an extension of the 

 margin of the enlarged upper layer, (Plate VIII, 

 figs. E,F, G) which folds itself up around the germ, 

 forming the sac of the so-called Amnios. You 

 may trace this outline here as it extends from the 

 lower part of the head around the navel, and fold- 

 ing backwards and upwards, and from behind the 



same, and from the side the same, and when these 

 folds unite upon the back, as they do in fig. F, the 

 germ is enclosed in another sac, though it is at the 

 same time surrouaded by the viielline membrane, 

 and the other substances contained in the shell. 



There is, therefore, a double protection to the 

 germ, and it is by the special developemt of this 

 sac that these birds are placed in a cavity full of 

 liquid, and during their development in this cavi- 

 ty, gills are formed similar to those which exist in 

 fishes. 



But there is soon another sac formed to pro- 

 tect once more the germ, rising in the form of a 

 vesicle from the lower and posterior part of the 

 body ; first, a small sac growing larger, and then 

 stretching (Fig. G) between the former sack and 

 the envelope of the yolk membrane, so as to sep- 

 arate both and to extend all around the germ, 

 forming another sac around it, the so-called Allan- 

 tois. And at this period the germ is enclosed within 

 two sacs ; first, one formed from the extension of 

 its own margin, and then another which rises as 

 a vesicle from its lower cavity ; and within these 

 envelopes, the young chicken is developed within 

 the hard egg- shell. Now in the Mammals, we have 

 a somewhat different adaptation. The same pro- 

 cess is at first going on, the same sacs enclose the 

 germ. Suppose only the enclosing membrane of 

 the egg not to grow hard, but to remain (Plate IX 

 fig. K) membranous ; and then we have the differ- 

 ences which characterize Mammalia, in which the 

 egg remains attached to the maternal body by a 

 peculiar development of the blood-vessels of the 

 Allantois ; when in this connection the germ un- 

 dergoes all its metamorphoses before the young is 

 born. 



All the difference which exists between these ani- 

 mals and the lower ones of the same type which 

 lay eggs, is only this that in the lower classes the 

 egg is laid with a hard envelope which is cast af- 

 terwards; while in the Mammalia the envelope of 

 the egg goes on growing within the germ, and is 

 not cast until the young is born. Other differences 

 do not exist. But these changes go on to produce 

 the great differences which we have noticed among 

 the different classes. Unfortunately, up to the 

 present day, Embryologists have been satisfied to 

 have traced the first outlines of the germ, and have 

 never considered it of any importance to trace the 

 changes which the very young undergo to assume 

 their final form. It struck me that it might be of 

 some interest, and during the last Spring, I opened 

 several eggs, at rather a late period, to see what 

 changes they would undergo. And I was surprised 

 to find that the first forms which are developed 

 are not those which will be permanent. Indeed, to 

 say it in a few words, that for instance, when the 

 legs and wings of birds are formed, they are not 

 formed under the shape of legs or wings. When 

 the paws of Rabbits are developed they are not de- 

 veloped with their fingers as they finally grow, but 

 appear first under the form of fins. 



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