DEVELOPMENT OF PHRYGANIDS. 569 



a very fair idea of it can be obtained by comparing PI. 

 XXXVIa figs. 3 and 4. 



The ventral plate has a characteristic dome-like shape, so 

 that its outline is strongly arched, both in cross and longitu- 

 dinal sections. 



At this stage the ventral plate is not much longer than it is 

 broad. The cells composing it are columnar, with well-defined, 

 rounded outer ends ; the inner ends, on the contrary, are faintly 

 marked, and send delicate, pseudopodial filaments into the 

 yolk. The protoplasm is very faintly granular ; it is also vacuo- 

 lated, and especially in the vicinity of the nuclei. Bobretzky^ 

 speaks of vacuoles in the endoderm cells of Oniscus, which 

 possibly may be of the same nature. 



During this stage of development certain nuclei present 

 very striking and characteristic appearances. Although most 

 abundant during this stage they may be observed to occur 

 with varying frequency up to the time of the revolution of 

 the embryo. The most noticeable but less frequent form is 

 that which has a very deeply-stained margin, and contains a 

 highly-refractive, non-stainable substance, in the centre of 

 which may occasionally be observed fine granules, scattered 

 about irregularly, or so concentrated as to form a kind of cen- 

 tral axis (PL XXXVIc, fig. 33). Another variety is seen iu 

 the nuclei represented in PI. XXXVIc, fig. 35) : they are 

 sharply limited, and contain very darkly stained filaments, 

 which at first sight appear like granules, but by careful focus- 

 sing with a highly-magnifying lens one can distinguish a con- 

 tinuous network of fibres, which in optical section look like 

 granules ; they are in marked contrast with the highly refractive 

 and non-stainable substance which constitute the remainder of 

 the nucleus.^ A third variety is seen in the great majority of 

 cells which constitute the embryo ; they stain deeply and uni- 



' N. Bobretzky, " Zur Embryologie des Ouiscus murarius," ' Zeit. fiir wiss. 

 Zool.,' Bd. xxiv, 1874. 



- Compare H. riemming, ' Zellsubstauz, Keru und Zelltbeilung,' Leipzig, 



1882. 



