Ko. 1431. BREEDiyC HABITS AXD EGfi OF riPEFISII—dVDGER. 485 



undergone considerable division in horizontal planes, as is shown b}' 

 the number of cells tilling the segmentation cavity. The large nuclei 

 are in the spireme stage, and in the left marginal cell there are two 

 large vacuoles. 



The high-arched type of this stage is shown in fig. 58, a derivative 

 of a structure like tig. 58, Plate Vlll. The surface falls steeply into 

 the outer periblast {<>. />.), the cells are all rounded and have small 

 nuclei. Very interesting are the two cells which are incompletely cut 

 off from the central periblast [e. p.). Scattered yolk granules are 

 found in some of the cells. The mitotic figures indicate that division 

 into the next stage has begun. 



In fig. 59 we have an example of the thick-ended type. The section 

 is slightly to one side of the center, and shows one cell just free and 

 another not yet cut out from the thick yolky periblast. Note the vacu- 

 oles which help to delimit cells. In the central section the small segmen- 

 tation cavity {s. c.) becomes somewhat larger. The outer cells are 

 flattened on the exterior, and the whole structure is very like fig. 55. 



STAGE OF ONE-HUNDRED-TWENTY-EIGHT SURFACE CELLS. 



The normal gently arched type is represented in tig. 60, a nearly 

 central section of a blastoderm of this stage. The central periblast 

 (c.jk) is here thick and fairly well delimited from the 3'olk below. Of 

 especial interest are the cells in the act of being cut out of it into the 

 segmentation cavitv. Very notal)le is the agency of vacuoles (v.) in 

 this process. The cell next to the right marginal cell has in its lower 

 part a nucleus, the first met with in the periblast region. 



Fig. 61 is an example of the fiat-arched type. The central periph- 

 eral cells, like those of the preceding stage, have undergone more 

 division than their fellows. The periblast at the left is reduced to a 

 mere line; at the right it is thicker and so tilled with yolk that one 

 can tind no line of separation save where the whole has come away 

 from the yolk. 



The round-arched type finds a good illustration in fig. 62. There 

 are three points of interest in this section: the presence of vacuoles, 

 which help to separate the right marginal cell from the ^^Rand;"' the 

 cell near the center still adherent to the central periblast, and. with its 

 neighbors, having some yolk particles in it; and two pairs of neigh- 

 boring cells with spindles at right angles to each other. These last 

 illustrate the exceedingly irregular segmentation in the pipefish Qgg. 



Fig. 63 is a nearly central section through a blastoderm intermedi- 

 ate between the normal and the thick-ended types. It is sharply 

 marked off from the outer periblast, which it overhangs on the right. 

 The segmentation cavity is reduced to the interstices between the 

 cells. All alo g the germ basis, in all the sections, cells are being- 

 cut out and the periblast la3^er left behind. An especially interesting 



