222 
BULLETIN OP THE UNITED STATES FISH COMMISSION. 
(see cut Fig. 4), starting from «, proceeds in tlie direction of the arrows. After the 
subgeriuinal cavity is formed the cell growth continues round the periphery in the 
same general direction (arrow) and a randwulst is established. In the region of the 
"Cb 
Fig. 4. — Diagrammatic section through a blastoderm to illustrate Gdtte’s account of the 
development of the randwulst and germ ring ; j). p., post, jjole ; a. p., ant. i)ole. ; m. to m., 
germ ring; s. c., segmentation cavity ; 2>., periblast. 
randwulst there is no differentiation of layers, but the cell growth continuing cen- ' 
tripetally produces the layer m-vi, sharply marked olf from the layer above. In a > 
somewhat later stage the differentiation of the under layer m extends back into the i 
randwulst itself, in the direction of the dotted line. 
Subsequent observers have either followed Oellacher’s account (IJoffinanu, 17) or i 
have been content to describe the main fact of an ingrowth from the edge of the bias- ■ 
todisc, witliout entering into a discussion of the randwulst (Heuuegny, 18; Agassiz 1; 
and Whitman, 1 ; Cnuniugham, 8). || 
The consideration of the germ ring as a part of the gastrnla will be found in a 
later section. f 
IV. NOTOCHORD, MESODERM, ENTODERM. 
Formation of embryonic shield . — After the germ ring is completed the growth of 
the blastoderm round theymlk is continued with much greater rapidity than in earlier i 
stages. In a stage 5 hours later than Fig. 35, PL xcii, two-thirds of the yolk is inclosed, I 
Fig. 36, PI. xcii. In the spreading of the blastoderm, the posterior pole or tail end of the ! 
embryo remains as a comparatively fixed point, while the anterior pole, a.p., Fig. 35, ; 
travels rapidly in the direction of the arrow. The constant position of the oil globule ; 
with respect to the early blastodisc enables one to j udge of this movement with certainty. ' 
During the growth of the blastoderm the embryonic area in the region of the pos- 
terior pole becomes sharply marked off' from the rest of the blastoderm. The area 
thus marked off' is commonly known as the embryonic shield. The manner in which j 
it develops is brought out in a comparison of Fig. 34, PI. xcii (20 hours), with Fig. 37, j 
PI. XCII (25 hours). As the anterior pole of the blastoderm travels round the yolk, [jj 
and the extra-embryonic region is thus increased, the head end (h. e., Fig. 34) of theii 
future embryo travels in the same direction, and in this way a roughly triangular areal i 
(embryonic shield) is established which projects into the surrounding extra-embryonic | 
region. The outline of the embyronic shield in Fig. 37 is indicated by the letters e. e. s. | 
In this figure the embryo itself is distinctly marked out as a median longitudinalj| 
thickening. A stage intermediate between Figs. 34 and 37 would show a clearly diff'erj !] 
entiated shield, though less extensive than that of Fig. 37, without the median j' 
thickening. li; 
