.EMBRYOLOGY AND LARVAL DEVELOPMENT OF TELEOSTEAN FISHES. 413 



periphery of the blastoderm. From this pole, which is the posterior or embryonic pole 

 of the blastoderm, a broad tongue of cells several layers in depth grows forward into the 

 subgerminal cavity. While the germ ring is becoming differentiated the blastoderm 

 gradually increases in size by centrifugal growth. After the germ ring is completely 

 differentiated the growth of the blastoderm round the yolk continues more rapidly 

 than in the earlier stages. The broad tongue of cells growing forward from the germ 

 ring at the posterior pole of the blastoderm becomes longer and gradually assumes a 

 roughly triangular form. In this manner the embryonic shield becomes distinctly out- 

 lined. As the embryonic shield increases in size a thickening representing the axis of 

 the future embryo occurs along its anteroposterior axis. As soon as this linear thick- 

 ening occurs an embryonic and an extra-embryonic area may be distinguished within 

 the embryonic shield. Figures 7 and 8 illustrate two successive stages in the differentia- 

 tion of the embryonic shield and the embryonic axis. The linear thickening advances 

 anteriorly from the posterior pole of the blastoderm but the differentiation of the embry- 

 onic axis begins in the head region and advances posteriorly. The embryonic area soon 



«T.._ ../... , 



CYPRINODON VARIEGATCS. 



Fig. 7. — Ees with blastoderm showing germ ring, gr, 

 fully difTercntiated and an early stage in differen- 

 tiation of embryonic shield, Es; pp. posterior pole 

 of blastoderm. 



Fig. 8. — Egg showing later stage in differen- 

 tiation of embryonic shield; ea, embryonic 

 area; EEA, extra-embryonic area. 



becomes broader in the anterior or head region than in the posterior region. In surface 

 view the embryonic area now presents a more or less regular spatulate form. During 

 the formation of the embryonic shield and the differentiation of the embryonic axis the 

 growth of the blastoderm round the yolk advances very rapidly. Before the embryo 

 is well differentiated (fig. 8) the blastoderm covers more than three-fourths the surface 

 of the yolk. Before segmentation of the embryo becomes apparent the blastoderm has 

 grown completely round the yolk and the blastopore is closed (fig. 9). 



The closure of the blastopore occurs less than 24 hours after fertilization. At this 

 time the embryo extends less than halfway round the circumference of the yolk. It 

 is relatively short and thick and somewhat irregular in outline. There is as yet no 

 evidence of pigmentation and the embryo is almost transparent. 



Soon after the closure of the blastopore relatively large melanophores appear 



sparsely scattered on the surface of the embryo and throughout the extra-embryonic 



blastoderm. Yellow chromatophores appear somewhat later. The latter never become 



numerous in the extra-embryonic blastoderm but soon become more numerous on the 



97867°— vol 34—16 27 



