498 



COMPARATIVE ANATOMY 



CHAP. 



yields the epithelium of the mid-gut in a way which has not been more closely 

 observed. Other observers, again, maintain that the whole enteric epithelium is 

 formed from the proctodseum and the stomodseum. 



It would perhaps be worth while to investigate whether the endoderm does not 

 arise from the yolk cells by a kind of micromere formation, in a way similar to that 

 in which the ectoderm is produced in many animals whose eggs contain much 

 nutritive yolk (cp. p. 124, etc.) 



The heart arises out of two lateral originally widely separated rows of mesoderni 



B 



Fig. 353. A and J5, Hydrophilus embryos with the rudiments of the extremities (after 

 Holder). In the somewhat older embryo, J5, the rudiments of abdominal feet, which disappear 

 later, are distinctly seen, a, Anal aperture ; an, antenna ; g, rudiment of the ventral ganglionic 

 chain ; m, oral aperture ; md, mandible ; mx\, first maxilla ; mx-2, second maxilla (rudiment of the 

 lower lip) ; pi, p*, jps, thoracic legs ; p, p^ y p?, p, rudiments of the extremities of the first, second, 

 fourth, and sixth abdominal segments ; st, stigmata ; vk, procephalon. 



cells. Each row, by sending out muscular processes, forms the side of a groove. The 

 two sides move towards each other, uniting later in the middle line to form the 

 cardial tube. 



The brain arises, like the ventral chord, as two lateral ectodermal thickenings 

 (neural plates), which remain separate from each other for a considerable time, but 

 from their first appearance are continuous with the lateral strands of the ventral 

 chord. Besides this a second ectodermal invagination on each side takes part in its 

 formation. 



Opinions as to the development of the eyes in insects still differ very much. 

 According to recent observations it appears probable that both the compound and the 

 simple eyes proceed from invaginations of the ectoderm, which become constricted off 

 as vesicles, and only secondarily become connected with the optic portion of the brain 

 (ganglion opticum). 



The first appearance of the sexual organs is not yet sufficiently investigated. 



