It has generally been maintained that the primitive streak and groove become wholly converted into the dorsal portion of the trunk of the embryo, _i.e._ into the posterior part of the medullary plate and subjacent structures. This view appears to me untenable in itself, and quite incompatible with the interpretation of the primitive streak given above. To shew how improbable it is, apart from any theoretical considerations, I have compiled two tables of the relative lengths of the primitive streak and the body of the embryo, measured by the number of sections made through them, in a series of examples from the data in Gasser's important memoir (No. 127). In these tables each horizontal line relates to a single embryo. The first column shews the number of somites, and the second the number of sections through the primitive streak. Where the primitive streak becomes divided into two parts the sections through the two parts are given separately: the left column (A) referring to the anterior part of the streak; the right column (P) to the posterior part. The third column gives the number of sections through the embryo. The first table is for fowl embryos, the second for goose embryos. +--------+--------------+----------+ | No. of | No. of | No. of | |Somites.| sections | sections | | | through | through | | | the | the | | | Primitive | Embryo. | | | Streak. | | +--------+--------------+----------+ | 0 | 29 | 7 | | 0 | 45 | 10 | | 0 | 39 | 23 | | 2 | 30 | 30 | | 4 | 30 | 30 | | | A P | | | 5 or 6 | 10 + 17 = 27 | | | 8 | 12 + 20 = 32 | 48 | | 12 | 13 + 10 = 23 | | | 14 | 9 + 12 = 21 | | | 18 | 10 + 7 = 17 | 70 | | | 8 + 4 = 12 | | | | 8 + 3 = 11 | | +--------+--------------+----------+ +--------+--------------+----------+ | No. of | No. of | No. of | |Somites.| sections | sections | | | through | through | | | the | the | | | Primitive | Embryo. | | | Streak. | | +--------+--------------+----------+ | 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 | | +--------+--------------+----------+