ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 601 



gradually curve towards the ventral surface and arc distinctly four- 

 jointed. 



The cephalic plate is bilobed ; a bilobed labruni has appeared 

 and the stornodreuni is faintly indicated between the rudiments of the 

 ckeliceras. The head and tail nearly meet dorsally and the caudal 

 plate has given rise to six meta somatic segments. The ectoderm 

 along the ventral mid-line is thinner than at the sides, and it is from 

 the thickened lateral bands that the nerve-ganglia are formed, one at 

 the base of each of the appendages ; those belonging to the cheliccra3 

 will soon disappear. The mesoderm is absent in the middle lino, 

 but laterally it splits into somatic and splanchnic layers, and is 

 d'vided up into segments. 



In the fourth period the reversion takes place. The tail gradually 

 becomes pointed aud much shorter ; the terga, which have appeared 

 in the mesosomatic segments grow dorsally, and the tail gradually 

 separates from the head on this surface. 



As the terga grow this process goes on till at the end of this 

 , stage the ventral surface is bent upon itself, so that the tail is directed 

 towards the head in exactly the opposite direction to what it is at the 

 beginning of the period. The stomoda3um is deepening ; the procto- 

 dseum has appeared at the tip of the shortened tail, and gives oif a 

 diverticulum which becomes the stercoral pocket of the adult. During 

 reversion the ectodermic bands, which give rise to the ganglia, become 

 widely separated and allow some of the yolk to project, so as to form 

 a sort of yolk-sac, which, however, is soon absorbed. 



At the base of the cheliceraa certain cells become spongy and form 

 the poison-gland, probably by invagination ; the spinning glands are 

 indicated by masses of ectoderm near the anus. Later on the in- 

 vaginations to form the pulmonary sacs appear; the lamellae arise 

 from cells which become arranged in parallel lines. The mesoderm 

 grows dorsally and becomes segmented, corresponding to the terga ; 

 so that these are not derived, as Balfour held, from the yolk. The 

 author was unable to ascertain the details of the formation of the 

 heart, but agrees with Schimkewitsch that Balfour's statement that it 

 arises from a solid cord of cells is wrong. Just before reversion 

 commences certain large cells are seen along the sides of the body, 

 which have arisen from the yolk, and form the " primary entoderm." 



The fifth period, lasts up till the embryo is hatched. A deep con- 

 striction separates the prosoma from the mesosome, and the embryo 

 becomes still more flexed. The two posterior pairs of provisional 

 appendages are transformed into spinning mamillaa. 



A few days before hatching the embryo begins to unroll and 

 undergoes a moult, and when hatched is quite straight. The eyes 

 have appeared, and the trachea) are indicated as invaginations on the 

 ventral surface. 



3. Organogeny. — At the time of hatching, the alimentary tract 

 consists of an anterior and a posterior portion, the inner ends of 

 which abut on the yolk. The stomodjeum gives rise to pharynx, 

 oesophagus, and stomach, which are lined by a cuticle continuous with 

 that of the exterior. The proctodeum gives rise to the stercoral pocket, 



