686 WILLIAM PATTEN, 



would account for the statement of Ticliomerof, that the meso- 

 derm gives rise to the epithelial lining of the mesenteron. 



The Malpighian vessels (PI. XXX VIb, fig. 17) arise as three 

 SEPARATE PAIRS of hollow outgrowths from the inner blind end 

 of the proctodaeum, one dorsal, one lateral, and one ventral ; 

 the latter pair appears first, and is somewhat larger than the 

 others ; from this pair also a fourth pair originates close 

 to its points of origin. Further increase in the number of 

 vessels is not produced by fresh evaginations of the procto- 

 dfeum, but by budding from the first three pairs of Malpighian 

 vessels. 



Stage 6. — The time from the rupture of the embryonic 

 membranes to that of hatching constitutes a sixth period. 

 The most important changes which occur are — (1) the rupture 

 of the embryonic membranes resulting in the formation of the 

 dorsal organ ; (2) the revolution of the embryo within the 

 chorion ; and (3) the formation of the heart. 



At the beginning of this period the embryo assumes a peculiar 

 shape, due to the fact that the abdomen has become bent back 

 upon itself, a part of its ventral surface being now concave 

 instead of convex. When the embryo has reached the condi- 

 tion represented in PI. XXXVIa, fig. 14, the amnion and serosa, 

 which have become confluent or, at least, closely united, rupture 

 along the line of confluence, the free edges of the membranes 

 being reflected back upon the sides and dorsum ; the rapidity 

 with which they move towards the dorsal region indicating 

 their elasticity and the high state of tension to which they 

 have been subjected. They now concentrate on the dorsal 

 portion of the yolk to form a rosette-shaped dorsal organ, 

 which gradually disappears from the surface (PI. XXXVIa, fig. 

 15). A complete series of sections at successive stages (Pi. 

 XXX Vic, figs. 39, 40, 4£) shows beyond a doubt that the dorsal 

 organ sinks into the yolk and is absorbed. In the stages 

 immediately following the retraction of the membranes the 

 nuclei are very sharply defined, and are embedded in a clear 

 mass of protoplasm in which is soon formed a tubular 

 invagination nearly perpendicular to the dorsal surface of the 



