238 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



splm 



a pair of deep medially directed invaginations arises just behind the mandi- 

 bles which fuse with the T-shaped invaginations to form the tentorium. 

 The dorsal tentorial arms arise afterward as outgrowths from the anterior 



tentorial arms. The mandibular 

 apodemes arise at the 90-hour 

 stage as a pair of deep invagina- 

 tions near the middle of the inner 

 side of the mandibular base. The 

 lower flexor mandibular muscles 

 become attached to them. In- 

 vaginations appearing soon after 

 blastokinesis and proceeding back- 

 ward from either side of the base 

 of the newly formed hypopharynx 

 on the ventral side form the 

 salivary glands. Already one day 

 after blastokinesis the two invagi- 

 nations approach each other and 

 finally fuse near their mouths to 

 form the common salivary duct. 

 Just after the completion of 

 blastokinesis the median area of 



the floor of the buccal cavity immediately in front of the labium becomes 



thickened to form the hypopharynx. This area arises by fusion of the 



sternites of the three jaw segments. As the mandibles and maxillae 



move closer to the mouth, the hypopharynx 



becomes closely associated with them. 



In the 112-hour embryo the dorsomedian 



walls of the coelomic sacs of all segments of 



thorax and abdomen begin to grow medially 



beneath and along the provisional dorsal 



closure (Fig. 160). Two lateral bands of 



splanchnic mesoderm are thus formed. This 



splanchnic mesoderm is two or more cells deep, 



the outer, or dorsal, layer in contact with the 



provisional dorsal closure, and the inner thin 



layer with a single layer of nuclei at right 



angles to the longitudinal axis of the embryo. 



The outer layer gives rise to the longitudinal 



and circular muscles of the mid-gut ; the inner 



layer soon after blastokinesis separates from the dorsal layer to form the 



short-lived mesentery which bounds the circumintestinal blood sinus from 



the outside (Fig. 164, ih). 



Fig. 160. Locusta. 

 abdominal segment of 112-hour embryo. 

 ibl) Blood cells, (cbl) Cardioblast. (cod) 

 Upper and lower portions of dorsal coelome. 

 (dc) Provisional dorsal closure, (ect) Ecto- 

 derm, (fire) Germ cells, (hs) Horizontal 

 septum, {mus) Lateral myoblast plate. 

 (sin) Lateral blood sinus, {splm) Splanchnic 

 mesoderm. 



ab.] 



Fig. 161. — Locusta. Lon- 

 gitudinal section of embryo 

 during blastokinesis showing 

 pleuropodium. {ab 1) First 

 abdominal segment, {mes) 

 Mesoderm, {th 3) Third 

 thoracic segment. 



