Book - A Text-book of Embryology 7
CHAPTER VII. THE FURTHER DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY
Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.
Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.
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Having traced the growth of the germ to the time when the body of the embryo becomes definitely diiferentiated from the embryonic a})|)endages or fetal membranes, the development of the individual organs and tissues may be taken up. The discussion of this latter subject, especially of that part of it pertaining to the structures on the exterior of the body, involves a consideration of the external form of the embryo and fetus during the successive stages of growth.
In the preceding chapters it was pointed out that the cells of the segmented ovum arranged themselves in such a manner as to form a hollow vesicle, the blastodermic vesicle (Plate I.) ; that this vesicle, having at first a single-layered wall, came to consist of two layers of cells, the ectoderm and the entoderm ; and that, finally, a third, intervening layer, the mesoderm, made its appearance. It was shown, further, that the thickened portion of the vesicle wall, the embryonic area, became more and more differentiated from the remainder, and that, by certain processes of folding, this area was made to assume the definite form of the embryonic body, while from the other parts of the vesicle-walls the fetal membranes were produced (Plate II.). It may be well to remind the reader again that when the body of the embryo has become closed off from the fetal membranes, this body is an irregularly tubular structure whose walls are the somatopleure and whose enclosed space is the body-cavity, and that within it are two other tubes, a larger, the gut-tract, formed by the splanchnopleure, and a smaller ectodermic tube, the neural canal.
While, as a inattor of coiivcniciKjo, tho (]('scri]>tion of the individual organs is taken up after tracing the course of development to this stage, it should he borne in mind that the rudiments of some of them are already distinguishable before the germ-layers become infolded to form the IxKly-wall and the gut-tract. It will facilitate a comprehension of the gi'ueral principles concerne<l in the origin of the different |>Jirts of the body to R»fer to the tabulated statement of the derivativi»s of the three primary germ-layers as presenteil in Chapter III.
In ciMisidering the external form of the prcnluct of conception, one may adopt the classification of I lis, referred to in the first chapter. This author divides the jHTiod of development into thn»i» stages, of which the ///W, the stage of the onuiuor the blastodermic stage, comprises the first an<l second we^ks of inira-uterine gri»vth ; the Htroiufy the stage of the «BihiTO« extends frimi the seci>nd to the fifth wi»ek ; and the nhtnL or tte fetal stage, includes the time bi^twwn the fifth w<^k aud the end of gestation.
THE STAGE OF THE OVUM.
Durii^ :ht:* t'»rtnii:lit allotteil to this first staire of develop invut nxMT chie various changi»< l\v whirh the impn»gnateil
•\'im H'»tui:>f^ th-.' fi^rm of a hollow <phen\ de<ignat«»<l the
fnoi-vtiiiii- 'i* ■^i:i>ciHK'rTnir vehicle. Tlu* M-rir^of tnin^torma >Mi> wa^ u»'ii Fi"i*'rtb^il in Chapter II. In ihi> pla«v it will
'^ ^ttiKi:««iii I' r'vr c.» tht* external ch:inictrr< of the blaston Fisr^. 4!>», whi.*h ri*pre<tMit
was estimated tu be alitm