EMBEYOLOGY OF THE SEA BASS. 
223 
During tlie growth of the blastoderm the part of the germ ring not included in the 
embryonic shield, extra-embryonic part of ring, grows thin and its cells become flat- 
j tened, as over the rest of the uon-embryonic area. The change which takes place in 
this part of the ring between 20 and 35 hours is gathered from a comparison of Fig. 48 
j (P objective) with the older stage, Fig. 51 (D objective). 
General structure of emhri/onic shield . — The ectoderm over the whole region forms 
a thickened plate, passing at the edge of the shield into the thin ectoderm ofthenon- 
embryonic area (Fig. 53, PI. xciv, transverse section through shield). The ectoderm 
plate is at first not thickened in tlie median line; on the contrary, just above the 
developing notochord it is somewhat thinner than elsewhere (Fig. 50, PI. xciv). The 
ectoderm cells are polygonal, except the epidermic stratum, and the layer is every- 
where clearly marked off, often actually separated from the under layer or primitive 
1 hypoblast pr. h., Pig. 50). 
■ The primitive hypoblast over the greater part of the shield consists, as in earlier 
stages (Fig. 44, 1^1. xoiii), of two strata of flattened cells. The strata ai-e quite distinct, 
except in the middle line (Fig. 50, PL xciv), where the cells are closely interlocked. The 
fusion which thus early appears in the primitive hypoblast along the median line is 
i! the first step in the formation of the notochord. At the lateral edge of the shield, 
|| where the primitive hypoblast ends abruptly, the two layers are not distinct (Fig. 53, 
PI. xciv), nor are they recognizable in the immediate neighborhood of the posterior 
I edge, where the primitive hypoblast bends round into the ectoderm (Fig. 49, PI. xciv, a 
1 transverse section through the extreme posterior edge of the shield). As will be seen 
’ in Fig. 37, PL xcii, this edge of the embryonic shield exhibits a ring-like thickening con- 
i| tinnons with the extra-embryonic germ ring, and the section given in Fig. 49, 1^1. xciv, 
i' shows that the thickening is located in the under layer. 
jl In the anterior region of the shield the primitive hypoblast is not divisible into 
!j two layers. Fig. 52, PL xciv, gives a longitudinal section, a little to one side of the 
median line, through a shield in which the embryo had just begun to be marked out as 
( a linear thickening. Anteriorly the primitive hypoblast ends in a mass of polygonal 
I cells, u. m., which long occupy this position, just in front of and ventral to the future 
fore-brain. From this mass as far back as the point x, in other words, throughout the 
jj extent of the future brain area, the cells of the under layer are polygonal, and only 
! indistinctly show two layers. Most, if not all, of the primitive hypoblast in the brain 
: region becomes converted into mesoblast exclusively (head mesoblast ; no somites). 
I Differentiation of the embryo and formation of notochord, etc . — The opaque linear 
j streak which marks the body of the embryo, and which is shown in a somewhat 
1 advanced condition in Fig. 37, PL xcii, is in its first stage due almost exclusively to a 
I thickening of the ectoderm. This thickening, by which the neural chord is formed, 
I begins in the future head region of the embryo, which is thus marked off before the 
j rest of the body. But in an hour’s time the posterior region (truidc) of the shield 
has also acquired its median thickening (Fig. 53, PL xciv, and Fig. 54, PL xcv, traus- 
i verse sections through the trunk). The start thus obtained by the brain keeps it, 
! however, from the beginning thicker than the spinal chord (Fig. 30, PL xcii, and 
longitudinal sections. Fig. 53, PL xciv, and Pig. 55, PL xcv). For the present the 
neural chord may be passed over with this brief description of its origin. 
By the time the neural chord has begun to form in the trunk region, the forma- 
: tion of the notochord and secondary layers has also commenced, by which the median 
