EMBRYONIC ENVELOPES, DORSAL ORGANS, BLASTOKINESIS 59 



stom 



dorsal organ of Hirschler. It differs in one respect. In the bee the 

 organ develops during the formation of the germ layers ; in Donacia it is 

 formed prior to that time. 



In the dipterous genus Sciara, Butt (1934) found a structure which 

 he designated as the "primary dorsal organ" that appears at an early 

 stage before the amniotic folds have closed over the ventral surface of 

 the embryo and consists of a thick saucer-shaped group of cells on the 

 dorsal side projecting toward the yolk. It reaches its greatest size at 

 the fifteenth hour and from then on decreases, until shortly before the 

 thirtieth hour it disappears entirely. The relation that the primary 

 dorsal organ bears to the embryonic envelopes has been discussed in a 

 previous section. 



THE SECONDARY DORSAL ORGAN 



The term "secondary dorsal organ" is applied to the contracted 

 serosa (in some cases the amnion) after the rupture. As has been 

 described in the account of the amnion 

 and serosa, the amnion forms the provi- 

 sional dorsal closure; the serosa, the 

 tubular transitional secondary dorsal organ 

 (Figs. 32, 35). It is well developed in 

 many Coleoptera (Hydrophilus, Dytiscus, 

 Leptinotarsa, Donacia, Tenebrio, Meloe), 

 the Orthopteroidea (Gryllotalpa, Phyllo- 

 dromia, Periplaneta), the Dermaptera, 

 Isoptera, Heteroptera, Homoptera, etc. 

 In the process of the formation of the 

 organ, the pavement epithelium of the 

 serosa becomes columnar, and the devel- 

 opment of the cylindrical structure pro- 

 ceeds from behind forward. In the 

 beginning a distinct lumen is present, but 

 this later disappears with a complete 

 degeneration of the cells in the yolk. 

 The destruction of the tubular organ takes 

 place at its posterior end and goes on as rapidly as it is being built up 

 at the anterior end (Fig. 35), so that at any one time the dorsal organ is 

 limited in extent. No part of it goes into the construction of the 

 definitive body wall. 



With respect to Leptinotarsa (Doryphora) Wheeler states that after 

 the rupture of the membranes they shift farther and farther upward 

 toward the middorsal line. When the embryonic area has so far extended 

 around the sides of the egg that its edges have nearly met, the remnants 



proct 



Fig. 35. — Tenebrio molitor. 

 Formation of the secondary dorsal 

 organ (do) from the serosa. The 

 amnion {am) forms a provi- 

 sional dorsal closure, {nc) Nerve 

 cord, iproct) Proctodaeum. 

 (stom.) Stomodaeum. (y) Yolk. 

 (From Soling.) 



