134 THE DEVELOPMENT OF 



Balfour apparently confounded this with an epibolic 

 gastmlation, for ('8O, p. 99) he seems to think that the 

 closure of the blastoderm is synonymous with the closing 

 of the blastopore, hence in these cases the blastopore 

 would be situated on the dorsal and not on the ventral side 

 of the ovum. Again (p. 378), he snys "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 it does not 

 occupy in any gastrula type so far dealt with. This fact, 

 coupled with the consideration that the partial segmenta- 

 tion of Scorpio can be derived without difficulty from the 

 ordinary Arachnidan type, seems to show that there is no 

 true epibolic invagination in the development of Scorpio." 

 That he nevertheless adhered to the first of these rather 

 conflicting ideas is seen from his statement a year later 

 ('81, p. 282) that "the epibolic gastrula of the scorpion, 

 of Isopods and of other Arthropods, seems also to be a 

 derived gastrula." Ayers holds a similar view for he says 

 ('84, p. 261) : In Scorpio, Mysis and Oniscus, the blas- 

 topore is dorsal in position." 



This view seems to me totally erroneous. If true, it 

 introduces some wonderful differences into the arthropods 

 and makes it impossible to trace close homologies between 

 forms as closely related as Mysis and Peneus ; for in the 

 latter case the blastopore is certainly ventral in position, 

 if we can trust Haeckel's figures. Unfortunately, we know 

 almost nothing about the inner germ layers of Scorpio, but 

 the little that Metschnikoff tells us ('71) is apparently in 

 accord with Bobretzky's account of Oniscus ('74) and since 

 the latter is much more fully described and moreover was 

 studied by sections, it may be taken to represent these 

 meroblastic crustacean ova. 



When the blastoderm of Oniscus covers about a third of 

 the egg, lower layer cells appear between it and the yolk 



