| No. of Somites. | No. of sections through the Primitive Streak. | No. of sections through the Embryo. |
|---|---|---|
| 0 | 10 | 4 |
| 0 | 28 | 5 |
| 0 | 44 | 12 |
| 2 | 36 | 32 |
| 4 | 24 | 42 |
| A P | ||
| 9 | 10 + 10 = 20 | 61 |
| 14 | 8 + 10 = 18 | 68 |
| 17 | 8 + 5 = 13 | |
| 22 | 9 + 6 = 15 | |
| 26 | 6 + 5 = 11 |
An inspection of these two tables shews that an actual diminution in the length of the primitive streak takes place just about the time when the first somites are being formed, but there is no ground for thinking that the primitive streak becomes then converted into the medullary plate. Subsequently the primitive streak does not for a considerable time become markedly shorter, and certainly its curtailment is not really sufficient to account for the increased length of the embryo—an increase in length, which (with the exception of the head) takes place entirely by additions at the hind end. At the stage with fourteen somites the primitive streak is still pretty long. In the later stages, as is clearly demonstrated by the tables, the diminution in the length of the primitive streak mainly concerns the posterior part and not that adjoining the embryo.
General history of the germinal layers.
The epiblast. The epiblast of the body of the embryo, though several rows of cells deep, does not become divided into two strata till late in embryonic life; so that the organs of sense formed from the epiblast, which are the same as in the types already described, are not specially formed from an inner nervous stratum. The medullary canal is closed in the same manner as in Elasmobranchii, the Frog, etc., by the simple conversion of an open groove into a closed canal. The closure commences first of all in the region of the mid-brain, and extends rapidly backwards and more slowly forwards. It is completed in the Fowl by about the time that twelve mesoblastic somites are formed.
The mesoblast. The general changes of this layer do not exhibit any features of special interest—the division into lateral and vertebral plates, etc., being nearly the same as in the lower forms.
Fig. 105. Diagrammatic longitudinal section through the axis of an Embryo Bird.
The section is supposed to be made at a time when the head-fold has commenced but the tail-fold has not yet appeared.
F.So. head-fold of the somatopleure. F.Sp. head-fold of the splanchnopleure.
pp. pleuroperitoneal cavity; Am. commencing (head-) fold of the amnion; D. alimentary tract; N.C. neural canal; Ch. notochord; A. epiblast; B. mesoblast; C. hypoblast.
