378 TRACHEATA. 



formed by a process which can be reduced to invagination : in other 

 words, there is no gastrula stage. 



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

 implies a modified gastrula, hut 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 l . 



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 Arachnidaii type (vide p. 99), 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 homo- 

 logous with, those of Chastopoda ; but the different modes by which 

 they arise in these two groups are very striking, and probably indi- 

 cate that profound modifications have taken place in the early 

 development of the Tracheata. In the Chastopoda 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 same 

 for all the Tracheata so far investigated, and is also very much the 

 same as for the Chastopoda. There is a division into somites, each 

 containing a section of the body cavity. In the cephalic section of 

 the mesoblastic bands a section of the body cavity is also formed. 

 In Arachnida, Myriapoda, and probably also Insecta, the body cavity 

 is primitively prolonged into the limbs. 



In Spiders at any rate, and very probably in the other groups of 



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. 



