MAN FROM THE FARTHEST PAST 
lens of the eye; the entoderm, of the lining of the long 
canal which is finally to make up the digestive as well as 
the speaking and breathing organs; and the mesoderm, of 
the bones, the muscles, the blood and lymph, lining of the 
body cavity, and the reproductive organs. 
In the initial stage, as we have seen, it is difficult if not 
impossible to recognize microscopically the differences 
between the nuclei of the germ-cells of different species or 
even of different orders of animals. But the eggs differ 
greatly in the quantity of the yolk in which the nuclei are 
immersed. With birds and reptiles, as we well know, the 
eggs contain much yolk, whereas with mammals, inelee 
ing man, the yolk is scanty. The development of the 
embryos associated with much yolk occurs outside the 
mother’s body and they derive all nourishment up to the 
time of birth from the egg. There naturally develops, as 
the means of nourishing the growing embryo, a channel of 
communication leading to the reservoir of food which the 
egg contains. This channel and reservoir are called the 
yolk-stalk and the yolk-sac. In the mammals we might 
well expect the absence of these appendages, for they are 
useless because the yolk is so scanty and the embryo almost 
immediately is attached to the parent’s circulation. Yet 
they exist, and persist for a long period, notwithstanding. 
(Plate 7A.) 
Other curious features in young mammalian embryos 
are the so-called gill slits. (See Fig. 8 in Plate 8.) These 
occur in the place corresponding to the gills of fishes. They 
are most marked in human embryos at the fourth or fifth 
week, and gradually are closed and modified into the 
organs of the face, so that they usually disappear before 
the end of the second month. 
Again, the human embryo hasa tail or coccyx, very plainly 
present as an exterior appendage during the second month. 
(See Plates 8 and g.) Though the infant generally retains no 
external vestige thereof at birth, sometimes (though rarely) 
the tail visibly persists throughout adult life. 
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