American journal of physiology . uditory E .s § § 0 Length Length Length I. 0 a, pit. 1) ^ OJ !fl in mm. in mm. in mm. Ph > cles. < ^ 1 2 3 4 36 56789 47 1011121314 64 15 65 16 67 19 68 20 Low 70 21 1 Present. 71 22 1 Approach ant. 72 23 3 Lie togetherant. Post, fopen 76 24 5-6 Lie togetherant. open 77 25 6 Lie togetherant. Post, fopen 81 26 8 Lie togetherant. Post. \open 86 27 8-9 Partly unitedant. Only lastopen Begin-
American journal of physiology . uditory E .s § § 0 Length Length Length I. 0 a, pit. 1) ^ OJ !fl in mm. in mm. in mm. Ph > cles. < ^ 1 2 3 4 36 56789 47 1011121314 64 15 65 16 67 19 68 20 Low 70 21 1 Present. 71 22 1 Approach ant. 72 23 3 Lie togetherant. Post, fopen 76 24 5-6 Lie togetherant. open 77 25 6 Lie togetherant. Post, fopen 81 26 8 Lie togetherant. Post. \open 86 27 8-9 Partly unitedant. Only lastopen Begin-ning 89 29 10-11 3 cerebral ves. Mostlyclosed Present 92 32 Present Rudiment 93 33 15 Mostlyclosed Present Deep, wideopen 98 38 trace 17 Mostlyclosed Present Deep, wideopen 100 41 18 102 43 19 Rudi-ment. Begin to close 107 46 27 Present Begin to close 109 48 27 Nearlyclosed Nearly closed 356 Charles Lincoln Edwards. The development of the primitive streak is shown in the followingcurve plotted from the above ~b fO 15 20 25 30 ^5 40^ DEVELOPMENT OF PRIMITIVE STREAK When considered in direct relation to the length of the incubationperiod there is a considerable individual variation in the growth of theembryo. This well known fact can be shown in an interestingmanner by some instances taken from the data given by Keibel andAbraham,^ but arranged by hours rather than in accord with theserial order of normal stages as given by these authors. Before the 20 hour stage, where the Normentafel begins, four fig-ures are drawn. The two for 9 hours of incubation have primitivestreaks, mm. and mm. long, thus being of the and 15 hourstages according to the scale I have constructed from Duval. Thetwo for 12 hours have primitive streaks mm. and mm. long 1 Keibel, F. : Normentafeln zur Entwicklungsgeschichte der Heft. Keibel, F., and Abraham, K. : Normentafel zur Entwicklungs-geschichte des Huhnes (Gallus Domesticus), Jena, 1900. Phy
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