File:The development of the chick; an introduction to embryology (1919) (14568795370).jpg

Identifier: developmentofchi1919lill (find matches)
Title: The development of the chick; an introduction to embryology
Year: 1919 (1910s)
Authors: Lillie, Frank Rattray, 1870-1947
Subjects: Birds -- Embryology
Publisher: New York, H. Holt and Company
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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F.B. 3u. ep. 3.^ S.I2. pr.str. Fig. 65. — Embryo of 12 s, from above, drawn in alcohol with re-flected Hght.au. ep.. Auditory epitheUum. Other abbreviations as before. to 25 somites; and then the mark was found to coincidewith the first somite of the series. In the next place it waspossible by similar means to mark out the topography of theembryonic head in the stage of one or two somites. Thus itwas determined that a mark made immediately in front of thefirst somite formed appeared later in the region of the otocyst; HEAD-FOLD TO TWELVE SOMITES 113 but this arises normally at the stage of 12-14 somites, a veryshort distance in front of the first somite of the series, which isthus shown to have the same position as the first somite formed.On the other hand, if one assumed that the first somite formed /f.F HL yVcA ^
Text Appearing After Image:
A.c.y. Vo.m. 3.1.p. Fig. 66. — The same embryo from beneath, drawn in alcohol with Abbreviations as before. reflected light. became the third or fourth of the series, it is clear that one wouldhave to make a mark some distance in front of the first somiteformed, to strike the place of origin of the otocyst. Marks madeon this theory were always found a considerable distance infront of the otocyst. Altogether a large number of experiments 114 THE DEVELOPMENT OF THE CHICK was made, the concurrent testimony of which was perfectlyconclusive. The somite formed in front of the first cleft is thus the firstin position of the definitive series and the remainder arise in suc-cession behind it. The formation of the somites therefore followsthe usual law of antero-posterior differentiation. There is always a stretch of unsegmented paraxial mesoblastbetween the last somite and the anterior end of the primitivestreak. The first four somites belong to the head, and enter into thecomposition of the

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