BUI,LETIN OF THE BUREAU OK FISHERIES. 



derms of i6 and 32 cells. This tendency may still be recognized in blastodorms of 

 64 cells. 



The successive acts of cleavage follow each other in rapid succession. Eggs showing 

 blastoderms in advanced stages of cleavage may be observed within three or four 

 hours after the time of spawning. Such eggs were usually observed between 9 and 1 1 

 o'clock p. m. 



Formation of the periblast. — During the early cleavage stages the marginal cells of 

 the blastoderm are not definitely limited peripherally, but are continuous with the thin 

 layer of protoplasm which remains at the surface of the yolk sphere. At the periphery 

 of the blastoderm this protoplasmic layer is concentrated to form a low ridge. This 

 ridge of protoplasm gives rise to the periblast (fig. 4, pb). As segmentation advances 

 nuclei become apparent in the periblast. These nuclei, as observed by Agassiz and 

 Whitman"^ (1884), are, doubtless, derived from the marginal cells of the blastoderm. 

 The cells at the margin of the blastoderm gradually become more definitely limited 

 peripherally until in the advanced stages of cleavage they are completely cut off from 



-pb 



BAIRDIELLA CHRYSURA. 



Fig. 5. — Egg with blastoderm of many cells, late cleavage 

 stage, surface view; pb, periblast. X 55. 



Fig 6.- 



-Egg with blastoderm of many cells, late cleavage 

 stage, lateral view; pb, periblast. X 55. 



the periblast (fig. 6). The blastoderm is now more or less dome-shaped and beneath 

 its central area may be observed a perceptible cleavage cavity. During the later cleavage 

 stages the periblast becomes somewhat more definitely outlined, increases somewhat 

 in width, and also sends a thin sheet of protoplasm centripetally beneath the cleavage 

 cavity. 



Formation of the germ ring and differentiation of the embryo. — While the marginal 

 cells of the blastoderm are becoming cut off from the periblast there appears a slight 

 thickening at the periphery of the blastoderm. This thickening represents an early 

 stage in the differentiation of the germ ring. It is caused primarily by the thinning 

 of the central area of the blastoderm and secondarily by the ingrowth (invagination) 

 of the marginal cells. The part played by invagination in the formation of the germ 

 ring and the embryonic shield is discussed at some length by Wilson (1889) in his paper 

 on the embryology of the sea bass.'' Evidence of invagination first appears at the 



^ Agassiz and Whitman: On the development of some pelagic fish eggs. Proceedings of the American Academy of Arts and 

 Sciences, vol. 20, 1S84. 



t> Wilson, H. v.: The embryology of the sea bass (.Serran-us atraiius). Bulletin of the United States Fish Commission, vol. 

 DC, 1889, p. 209-277, pi. Lxx-wm-cvn. 



