118 MALCOLM LAURIE. 



tant changes. As mentioned above, it has now separated into 

 two longitudinal bands. This separation does not extend into 

 the tail-segment (fig. 23, mes.), where the mesoblast remains 

 as a solid mass of cells somewhat thinner in the middle line. 

 The ccelomic spaces are now formed by a splitting of the meso- 

 blast in the somites. They are best seen in the posterior 

 somites (fig. 21, cm.), where the mesoblast is thin and forms 

 only a single layer on each side of the coelomic space. Further 

 forward (fig. 22) the mesoblast is thicker and the coelomic 

 space is not so well marked. 



The hypoblast has undergone very little change. It is still 

 visible in the tail as a solid mass (fig. 23, hy.m.), and spreads 

 under the ventral plate and a little way beyond its margin as 

 a single layer (figs. 21 — 23, hy.). The cells of this single layer 

 have large oval nuclei which stain less darkly than those of 

 the epi- and meso-blast. These nuclei are somewhat widely 

 separated from each other, and the cells seem to contain a 

 considerable amount of food- stuff. 



The serous membrane (figs. 21 — 23, s. m.) is by this time quite 

 separate from the egg all round. • It has attained its final 

 structure, the nuclei being enormously large ("05 mm.), flat, 

 and at a considerable distance from each other. As far as my 

 observations go I can confirm Blochmann^s statement^ that the 

 nuclei of the serous membrane divide directly without forming 

 any karyokinetic figures. As the serous membrane plays a 

 purely passive part in the future development it will not be 

 necessary to refer to it again. 



Stage H. — In the next stage (PI. XV, fig. 24-), which is 

 the last before the formation of the appendages, the embryo 

 consists of nine somites. The first is very much smaller than 

 the others, while on the second, which is the largest, a trace 

 of the appendages is just visible. The first six somites are 

 clearly distinguished from those further back, owing to their 

 sloping backwards and outwards, while the posterior ones are 

 at right angles to the axis of the embryo. 



^ " Ueber direkte Kerutbeilung in der Embryonalliulle dcr Skorpione," 

 ' Morpb. Jabrb.,' vol. x. 



