616 L. H. GOUGH. 



In transverse sections it otherwise presents a similar appear- 

 ance to that of more advanced stages. 



The cavity of the alimentary canal is Y-shaped ; the cells 

 composing its walls are high and cylindrical. The nuclei 

 chiefly lie at the end of the cell nearest to the cavity. A thin 

 cuticle has been secreted by these colls. 



The whole is surrounded by a thin layer of mesoderm cells, 

 which form a skin round the tube. The nuclei of these cells 

 have begun to elongate, and will probably form the ring- 

 shaped muscle found later on in this part. 



(7) The Heart. — The circulatory organs are at present 

 represented by the heart alone. The origin of the heart 

 seems to me to be the same as in Scorpions (Brauer [7]) 

 and other Arachnids. It is evidently formed by the ccelom 

 on both sides of the embryo meeting, leaving a space between 

 the walls on either side, which, although surrounded by 

 coelomatic walls, does not belong to the ccelom itself. The 

 heart causes a slight ridge on the surface of tlie embryo. 



(8) The Lungs. — At this stage the lung -books are just 

 beginning to make their appearance. They belong to the 

 first and second abdominal extremities. Laurie (31), however, 

 has located them on the first and third. They differ neither 

 in development nor in appearance from the lungs of other 

 Arachnids. 



(9) The Muscles. — Typical muscles are as yet nowhere to 

 be found. In the extremities, and also in the abdomen, we 

 find the cells which give rise to the future muscles. These 

 are distinguishable by their long, granular nuclei. Though 

 already spindle-shaped, they do not as yet stain with eosiu, 

 as the muscles in more advanced embryos always do. 



(10) The Genital Organs. — The genital organs have not 

 as yet begun to make their appearance. 



Stage V. 



The two illustrations (figs. 1 and 1 a) of the embryo at this 

 stage will give a very good idea of the general superficial 



