14 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



cases begin in the middle and extend caudally and finally to the head end. 

 The invaginated part is in the form of a tube which later flattens, obliter- 



"^""^ neurg ^m.cav. 



Fig. 14. — Cross section of germ band after completion of serosa {aer). {am) Amnion. 

 (am.cav) Amniotic cavity, (ect) Ectoderm, (il) Inner layer, {neur) Neuroblasts. 

 {neurg) Neural groove, (yc) Yolk cell, (ysp) Yolk spherule. 



ating the cavity (Figs. 13, 14). The flattened tube represents the inner 

 (or so-called ''lower") layer which differen- 

 tiates into a median, or middle, strand which 

 ends anteriorly and posteriorly in a cell 

 mass (the so-called "secondary entoderm") 

 and the two lateral mesoderm bands. To 

 what extent we are justified in considering 

 the longitudinal furrow as a gastrular invagi- 

 nation and in regarding the inner laj^er as 

 composed of a middle entoderm strand and 

 two lateral mesoderm bands is discussed in 

 Chap. VI of this text. 



Blastokinesis, Segmentation of the Em- 

 bryo, and Formation of Appendages. — When 

 the lateral and anterior amniotic folds have 

 formed, the embryo pushes still further into 

 the yolk. This shift in position with 

 respect to the yolk and change in length 

 is termed "blastokinesis" (Fig. 12). 

 Meanwhile there may now be recognized, 

 first, the protocephalon, i.e., the cephalic 

 lobes of the embryo comprising the labrum, 

 eyes, antennae, and usually the postoral 

 somite of the second antennae; and, second, 

 the protocorm, or the remaining part of the 

 embryo. Later segmentation occurs, 6 segments eventually forming in 

 the head, 3 in the thorax, and 11 in the abdomen with a telson in addition. 



pleur 



Fig. 15. — Ventral aspect of 

 embryo, (anl) Antenna, (cly) 

 Clypeus. {lb) Labium. {Ir) 

 Labrum. (m) Mouth, {md) 

 Mandible, {mx) Maxilla, {p) 

 Thoracic legs, {pleur) Pleuro- 

 liodium. 



