TRACHEATA. 457 



Efforts have been made to shew that the mesoblastic groove of Insects 

 implies a modified gastrula, but since it is the essence of a gastrula that it 

 should directly or indirectly give rise to the archenteron, the groove in 

 question cannot fall under this category. Although the mesoblastic groove 

 of Insects is not a gastrula, it is quite possible that it is the rudiment of a 

 blastopore, the gastrula corresponding to which has now vanished from 

 the development. It would thus be analogous to the primitive streak of 

 Vertebrates 1 . 



The growth of the blastoderm over the yolk in Scorpions admits no 

 doubt of being regarded as an epibolic gastrula. The blastopore would 

 however be situated dorsally, a position which it does not occupy in any 

 gastrula type so far dealt with. This fact, coupled with the consideration 

 that the partial segmentation of Scorpio can be derived without difficulty 

 from the ordinary Arachnidan type (vide p. 120), seems to shew that there 

 is no true epibolic invagination in the development of Scorpio. 



On the formation of the blastoderm traces of two embryonic 

 layers are established. The blastoderm itself is essentially the 

 epiblast, while the central yolk is the hypoblast. The formation 

 of the embryo commences in connection with a thickening of the 

 blastoderm, known as the ventral plate. The mesoblast is 

 formed as an unpaired plate split off from the epiblast of the 

 ventral plate. This process takes place in at any rate two ways. 

 In Insects a groove is formed, which becomes constricted off to 

 form the mesoblastic plate : in Spiders there is a keel-like 

 thickening of the blastoderm, which takes the place of the 

 groove. 



The unpaired mesoblastic plate becomes in all forms very 

 soon divided into two mesoblastic bands. 



The mesoblastic bands are very similar to, and probably 

 homologous with, those of Chaetopoda ; but the different modes 

 by which they arise in these two groups are very striking, and 

 probably indicate that profound modifications have taken place 

 in the early development of the Tracheata. In the Chaetopoda 

 the bands are from the first widely separated, and gradually 

 approach each other ventrally, though without meeting. In the 

 Tracheata they arise from the division of an unpaired ventral 

 plate. 



The further history of the mesoblastic bands is nearly the 



1 The primitive streak of Vertebrates, as will appear in the sequel, has no con- 

 nection with the medullary groove, and is the rudiment of the blastopore. 



