OF OECANTHUS AND TELEAS. 245 



The heart is formed in connection with the coalescence of the mesoderinic plates in the 

 median dorsal line and is to be detected as a distinct thin-walled tube opposite to, or 

 slightly in advance of, the anterior edge of the thickened ectodermic layer. It is formed 

 in the head region only after the yolk-sac has passed entirely within the body. When 

 the heart is partly formed, the mesodermic plates anterior to their point of union form in 

 the living embryo two pulsating membranes which, to judge from Dohrn's (15) observa- 

 tions on Gryllotalpa, differ in their formation from the homologous membrane in the 

 latter insect. I have not observed pulsations in the dorsal mesodermic layer after fusion of 

 the two plates, such as Dohrn has described for Gryllotalpa ; but before the coalescence 

 takes place the free edges of the lateral plates are thrown into wave-like motions at each 

 pulsation of the formed portion of the dorsal vessel. The pulsations originate from the 

 contractions of the segmental muscles in the posterior abdominal region, which drive the 

 corpuscular fluid forward through the heart to the point of bifurcation, from which the fluid 

 passes through the channels in the edges of the plates, and over the exposed surface of 

 the yolk between them. Each pulsation occupies l£ seconds in traveling from the tip of 

 the body to the umbilicus of the yolk-sac. The pulsations occur in series with intervals 

 of repose between them, which sometimes last for three or four minutes. The pulsa- 

 tions of each series follow each other regularly at the rate of one per second, so that while 

 one wave of pulsation is progressing through the free edges of the mesodermic plates 

 another one has originated in the posterior end of the dorsal vessel. 



The secretion of a cuticular covering by the ectodermic layer begins first on the ventral 

 surface of the body. The appendages appear to be simultaneously enveloped by the 

 secretion, which closely encases them ; however, soon after the closure of the dorsal wall 

 and the secretion of the cuticula in this region, the whole layer becomes distended 

 with a fluid and is thereby removed from the body. At the ends of the appendages the 

 cuticula is swollen into a bulb-like enlargement which allows a free movement of the tip of 

 the growing limb ; this is especially noticeable in the antennae, which at this time are rap- 

 idly increasing in length. The embryo soon comes to fill completely the cavity of the egg ; 

 the legs are now folded upon themselves, pi. 19 figs. 4 and 5 ; the antennae curve around 

 the end of the abdomen and reach nearly to the head on the dorsal side. The embryo has 

 attained its full size and is enveloped by two cuticular layers, — the primitive layer surround- 

 ing the body like a loose sac, and the secondary cuticula closely investing the hypodermis. 

 The outer layer shows only a few irregularities of surface, while the inner layer is pro- 

 duced into innumerable spines, bristles, and hair-like processes. (PL 19, figs. 14 and 15.) 

 The mouth parts have grown shorter and stouter, while the cuticula of the mandibles and 

 the inner lobes of the maxillae has become much thickened to form the biting mouth parts. 

 The yolk has been consumed during this interval of growth and the digestive tract is 

 completed in all its essential parts. The body walls have become thinner as the internal 

 organs acquired their relative proportions, so that they now consist of a thin hypodermal 

 layer surrounded by its tough cuticula. The food supply being exhausted, the embryo 

 bursts its membranous coverings and becomes free. 



The foregoing summary of the changes through which the embryo passes from the time 

 of revolution until it leaves the egg will be of service in properly connecting the following 

 detailed account of the development of the separate structures of the now complicated 



