148 EMBRYOLOGY 



represents a stage in the development of Pyrrhocoris, showing 

 the interior of the egg after a body has been formed by the union 

 of the sperm and egg-nuclei ; this body is about to undergo 

 division or segmentation, and the equatorial arrangement where 

 this will take place is seen. The tw-o polar bodies P^^, P„, after 

 having been excluded, are nearly , reincluded in the egg. 



The Ventral Plate. 



The next important change after the formation of the blastoderm 



is the partial detachment of a part of its periphery to become 



placed in the interior of the other and larger 



A ^'.-'='S;'W^??>^ portion. The way in which this takes place 



f .. yj will be gathered from the accompanying dia- 



i-Vii^^v.v;-:^:'-!^-'^ grammatic figures taken from Graber : a 



B ,-.-Jv.-vr";.--~ thickened portion (« I) of the blastoderm 



,'' .°#°'f 'V'jSkS^va becomes indrawn so as to leave a fold 



^S\^°2£^^i<?Ac (c (^0 fi-t each point of its withdrawal, and 



^_.__ these folds afterwards grow and meet so as 



z, ''^°:°' %''-^'^?^'v'^ *° enclose the thickened portion. The outer 



'W^^^^0^^ envelope, formed in part by the original 



^'^f'""""""""*^' > blastoderm and in part by the new growtli, 



Fio. 80.-Stages of the '' ^''^^^'^ .^^''^ '''''°''^ ^^ "^ )' *^^ "''^^^' ^^^'^'^ (^) °f 



euciosure of the veil- the coujoined 2iew folds being termed the 

 yentrli'^'piate''^' B c' '^^i^^i^^^ ' ^^^^ part withdrawn to the interior 

 d, folds of the bias- and covered by the serosa and amnion is 

 '—2t:::T^l ^-^l^d the ventral plate, or germinal band 

 amnion and serosa; (Aeunstreif), and becomes developed into the 

 fe'rosafv/'amnil.*^' future animal. The details of the withdrawal 

 of the ventral plate to the interior are very 

 different in the various Insects that have been investigated. 



One of the earliest stages in the development is a differentia- 

 tion of a portion of the ventral plate into layers from which 

 the future parts of the organisation will be derived. This 

 separation of endoderm from ectoderm takes place by a sort 

 of invagination, analogous with that by which the ventral plate 

 itself is formed. A longitudinal depression running along the 

 middle of the ventral plate appears, and forms a groove or 

 channel, which Ijecomes obliterated as to its outer face by the 

 meeting together of the two margins of the groove (except on the 



