Human physiology : designed for colleges and the higher classes in schools and for general reading . int in the egg. But there is a particular contrivance whichprevents it from actually touching the shell. It is held downby two very delicate ligaments e,e, connecting it with the whitelining of the shell. And you will observe, too, that the cica-tricula, or germ-spot, a, which is a collection of cells beginningthe process which is to form the animal, being lighter than theyolk is always at the top of it, in order to receive the warmthfrom the body of the bird as it sets upon its eggs. Besides a
Human physiology : designed for colleges and the higher classes in schools and for general reading . int in the egg. But there is a particular contrivance whichprevents it from actually touching the shell. It is held downby two very delicate ligaments e,e, connecting it with the whitelining of the shell. And you will observe, too, that the cica-tricula, or germ-spot, a, which is a collection of cells beginningthe process which is to form the animal, being lighter than theyolk is always at the top of it, in order to receive the warmthfrom the body of the bird as it sets upon its eggs. Besides allthis, there is at the blunt end of the egg, /, a bubble of airwhich is intended as an invigorating draught for the lungsof the young bird, preparatory to its bursting its shell. 210. When the processes preparatory to the formation of theanimal commence, the yolk itself is composed in part of cells,as represented in Fig. 71, A. In the midst of it there is agerminal spot, a, with a vesicle in it, b. This vesicle produces CELL-LIFE. 135 Succession of cells in the yolk before the animal is DEVELOPMENT OF CELLS IN THE YOLK DURING INCUBATION. a cluster of cells. But these cells, and those which in partcompose the yolk are temporary, and all disappear. Before,however, the cluster of cells in the germinal spot disappear,there are seen in the midst of them two twin cells. Thesemultiply ; and what is singular, they do it by doubling, so thatthere are successively 4, 8, 16, 32, &c. At length there is amass of them, like a mulberry, as at e, in B. This mass thensends off cells at its edges which makes a layer,/, all roundthe yolk as represented in C. A second layer, g, is formedinside of the first as seen in D. In the case of the higheranimals a third layer is added. 211. There is no formation of the animal yet. But now asingle large cell appears in the centre of the mulberry-shapedmass of cells, and from this begins the formation of the the other parts of the egg—t
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Keywords: ., bookauthorhookerwo, bookcentury1800, bookdecade1850, bookyear1854
