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 two polar bodies P^, P^, after 

 having been excluded, are nearly reincluded in the Qg^. 



I 



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 

 portion. The way in which this takes place 

 will be gathered from the accompanying dia- 

 grammatic figures taken from Graber : a 

 thickened portion {a h) of the blastoderm 

 becomes indrawn so as to leave a fold 

 (6* d) at each point of its withdrawal, and 

 these folds afterwards grow and meet so as 

 to enclose the thickened portion. The outer 

 envelope, formed in part by the original 

 blastoderm and in part by the new growth, 

 is called the serosa (e f ), the inner layer {g) of 

 the conjoined new folds being termed the 

 amnion : the part w^ithdrawn to the interior 

 and covered by the serosa and amnion is 

 toderm that form the called the Ventral plate, or germinal band 



commencement of the , . . 



amnion and serosa; {Keimstreif), and becomes developed into the 

 c, e, f, part of the f^^^^^^g animal. The details of the withdrawal 



serosa ; g, amnion. 



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 becomes obliterated as to its outer face by the 

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



Fig. 80. Stages of the 

 enclosure of the ven- 

 tral plate : A, a, b, 

 ventral plate ; B, c, 

 d, folds of the bias- 



fl 



