EMBRYOLOGY AND LARVAL DEVELOPMENT OF TELEOSTEAN FISHES. 93 



layer during the process of concentration are less apparent in the eggs of this species 

 than in the eggs of many other species of teleosts by reason of the extremely minute 

 size of the protoplasmic granules. The process of concentration occupies less than 

 one-half hour. The fully differentiated blastodisc (fig. i, bd.) comprises nearly all 

 the protoplasm contained in the egg. It is circular in outline and of nearly uniform 

 thickness throughout the central area, thinning out abruptly near the periphery. At 

 the periphery it thins out gradually into a very thin layer of protoplasm, which 

 continues to invest the yolk sphere. 



Segmentation. — The first act of cleavage occurs less than one hour after fertiliza- 

 tion. Later acts of cleavage follow each other in rapid succession. Blastoderms in 

 advanced stages of cleavage may be observed within four hours after fertilization. 



As the moment of cleavage approaches, one axis of the blastodisc becomes somewhat 

 longer than the other. The first plane of cleavage cuts the blastoderm at right angles 

 to the longer axis (fig. 2). The second plane of cleavage cuts the first at right angles. 



During the four-cell stage (fig. 3) the two axes of the blastoderm are approximately 

 equal. The third planes of cleavage cut the blastoderm approximately parallel with the 

 first (fig. 4). As the third act of cleavage occurs one axis of the blastoderm again 

 becomes distinctly longer than the other (fig. 4). Typically, the eight blastomeres 

 formed by the third act of cleavage lie in two symmetrical series of four cells each. 

 As the fourth act of cleavage occurs, the two axes of the blastoderm again become 

 approximately equal. The blastoderm now becomes more or less circular in outline 

 and approaches true radial symmetry more and more closely as cleavage advances. 



The first two or four blastomeres are usually approximately equal in size and quite 

 symmetrical. As the third act of cleavage occurs, symmetry is usually disturbed. 

 Early blastoderms of more than four cells show a marked lack of symmetry and 

 frequently some disparity in the size of the constituent cells. However, blastoderms 

 of 8, 16, and 32 cells are found occasionally which remain almost ideally symmetrical. 

 Beyond the 64-cell stage symmetry or lack of symmetry in the arrangement of the cells 

 is not easily observed. Blastoderms in advanced stages of cleavage usually appear 

 radially symmetrical. 



Formation of ike periblast. — The cells at the margin of the blastoderm are not 

 sharply limited peripherally, but remain continuous with the thin layer of protoplasm 

 at the surface of the yolk. As segmentation advances this layer of protoplasm 

 becomes concentrated at the periphery of the blastoderm into a somewhat flattened 

 protoplasmic ridge that gives rise to the periblast (fig. 5, pb). Before this ridge of 

 protoplasm has become fully differentiated, nuclei become apparent near the margin 

 of the blastoderm and gradually become distributed throughout the entire protoplasmic 

 ridge. The periblast nuclei as observed by Agassiz and Whitman," doubtless are 

 derived from the peripheral cells of the blastoderm. When fully differentiated the 

 periblast consists of a flattened syncytial ridge of protoplasm with nuclei apparently 

 like those of the cells in the blastoderm distributed throughout its entire extent. 



Until nuclei are present throughout the peripheral area of the periblast it remains 

 continuous with the peripheral cells of the blastoderm. As segmentation advances 

 further the peripheral cells of the blastoderm become completely cut off from the peri- 



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

 vol. 30. 1S84. 



69.571°— 18 7 



