Blood et al.: Embryonic development of Theragra chalcogramma 



213 



E 

 E 



O) 



c 



9 



C 

 CO 

 9 



s 



5.5 



5.0 



4.5 



4.0 



3.5 



"T 



O 7./C 

 D 6.7°C 

 A 3.8° C 



3.0 

 204 



248 



292 



336 



380 



424 



Total incubation time (h) 



Figure 3 



Mean hatch lengths at each sampling interval during the hatching period of 

 Theragra chalcogramma larvae incubated at 3.8°, 5.7°, and 7.7°C. Stage of de- 

 velopment at hatch is also shown. Shaded circle indicates overall mean length 

 for each temperature. Vertical bars are standard deviations; numbers indicate 

 sample size. 



plane. Cells form a rectangle with the four cells in 

 the center smaller than those at the corners of the 

 rectangle (Figs. 5D and 6D). 



16 cells (stage 5) The fourth cleavage is perpen- 

 dicular to the third; this is the last stage in which 

 cell division is restricted to the horizontal plane. 

 Most eggs have a square or rectangular block of cells 

 with four cells on each side; all cells are in contact 

 with yolk through this stage (Figs. 5E and 6E). 



32 cells (stage 6) Initially, the single layer of 

 cells has a flat, irregular square or rectangular 

 shape. Cell division continues in horizontal and 

 vertical planes, transforming the blastodisc into a 

 hollow cap of cells on the yolk resembling a rasp- 

 berry (Figs. 5F and 6F). Cells increase in number 

 but the size of the blastodisc remains constant. The 

 perivitelline space widens between yolk and chorion. 



Blastodermal cap (stage 7) The blastodisc 

 progresses through two steps: at first, cell size de- 

 creases from continued cleavage; cell material ap- 

 pears granular and the blastodisc resembles a flat- 

 tened dome on the yolk surface. Then, the base of 

 the cell mass sinks below the yolk surface; the 

 periblast extends beyond the equator of the blasto- 

 disc, giving the appearance of a "flying saucer" in 



lateral view (Figs. 5G and 

 6G). 



Early germ ring (stage 



8) The center of the blasto- 

 disc flattens and the periph- 

 ery (germ ring) thickens in 

 preparation to overgrow the 

 yolk (epiboly). The blasto- 

 coel, visible on one side of 

 the blastodisc, appears 

 grainy and pale (Fig. 5H). 

 The margin between blasto- 

 coel and blastodisc is indis- 

 tinct (Fig. 6H). 



Germ ring 1/4 around 

 yolk (stage 9) The blasto- 

 disc, now the embryonic 

 shield, expands as the germ 

 ring begins to overgrow the 

 yolk. The margin of the fu- 

 ture anterior end of the em- 

 bryo is slightly curved and 

 sharply defined. Cell mate- 

 rial covering the blastocoel 

 appears less grainy than in 

 the previous stage. After 

 preservation, this thin cellu- 

 lar layer appears concave in 

 lateral view. The germ ring 

 margin is thin and flattened, 

 extending 1/4 around yolk (Figs. 51 and 61). 



Germ ring 1/2 around yolk (stage 10) The 

 germ ring envelopes half the yolk and the anterior 

 margin of the embryonic shield is sharply curved 

 and thick (Figs. 5J and 6J). The beginning of neu- 

 ral development is visible; a neural keel extends 

 from the anterior margin of the embryonic shield to 

 2/3 its length (Fig. 5K). 



Germ ring 3/4 around yolk (stage 11) Head 

 and upper body region begin to differentiate but no 

 distinct brain lobes are apparent. Optic vesicles 

 develop. Prospective head and body mesoderm out- 

 lines the hour-glass shape of the developing embryo 

 (Fig. 7A). The notochord is visible ventrally. The germ 

 ring has progressed 3/4 down the yolk (Fig. 6K). 



Late germ ring (stage 12) Myomere differen- 

 tiation begins; separate myomeres are not visible. 

 The midbody expands dorsoventrally; prospective 

 head and body mesoderm forms a narrow outline of 

 the embryo. The blastopore is open and the germ 

 ring envelopes more than 7/8 of the yolk (Figs. 7B 

 and 8A). 



Early middle stage (stage 13) The blastopore 

 is closed. The notochord and 7-12 incomplete 

 myomeres are visible. Tail margin is indistinct and 



