76 BULLETIN OF THE 



two lateral elements has been developed, and the stomodreum has be- 

 come faintly marked (PI. III. fig. 16, PI. IV. fig. 23). The four pairs 

 of provisional appendages are now fully established, and the embryo has 

 increased in length till the head- and tail-lobes are nearly in contact ; 

 the dorsal region is, as a consequence, much reduced. Behind the 

 somites which bear the provisional appendages the tail-lobe has given 

 rise to at least six indistinct additional somites ; the terminal end of the 

 tail is much narrowed and is becoming more pointed. The swellings 

 produced by the rudimentary ganglia, at the bases of the appendages, are 

 further developed, and the median ventral furrow has increased both in 

 depth and in width. 



Balfour has given good figures and descriptions of the germinal layers 

 during the formation of the appendages. The mesoderm is of especial 

 interest at this time. Early in the protozonite stage it forms a con- 

 tinuous band, about as wide as the embryo, composed of a single layer of 

 cells extending the whole length of the embryonic band. About the 

 time the appendages begin to appear the mesoderm splits along the 

 median ventral line, thus forming two parallel bands, which remain 

 united, however, in the head and tail regions. The division of the 

 mesoderm into lateral halves is followed by an increase in the tliickness 

 of the resulting bands, each of which becomes split into a somatic and a 

 splanchnic layer. It is also at this stage that the mesoderm is divided by 

 transverse constructions into somites, each of Avhich contains a central 

 lumen. I am unable to determine from my specimens whether its divis- 

 ion into successive blocks precedes or fullows the appearance of the 

 lumen. In the growth of the appendages the somatic layer of the meso- 

 derm accompanies the outgrowing ectoderm, and forms a continuous 

 lining to its cavity. 



During this period the ectoderm has also increased in thickness, but 

 along the median ventral line it remains thinner ; from this it results 

 that there are two bands of thickened ectoderm corresponding to the two 

 deep bands of mesoderm. The ventral median depression previously 

 mentioned is at first due to the relative thinness of the ectoderm in this 

 region ; it is afterwards made more conspicuous by the further separa- 

 tion of the mesodermic bands. From the ectodermic bands are formed 

 the nervous ganglia. They are developed first in the thoracic region in 

 the form of swellings at the bases of the appendages, but by the time the 

 stage represented in Fig. 8 has been reached, they have also been formed 

 in the abdominal region. 



As already correctly maintained by Balfour, the segment of the cheli- 



