164 Embryonic Development of Chironomns 



gnatha it runs as a tube through the yolk ', In the less 

 complex cases of animal development, which are usually 

 chosen for elementary teaching, the entoderm arises by 

 invagination of the blastoderm (Sagitta, Amphioxus, 

 Echinoderms). Here there is little or no yolk. Where 

 yolk becomes abundant we get the modifications known 

 as e]3iboly, delamination, polar regression, &c. The con- 

 tinuity of the entoderm may be lost. Its cells may be 

 gorged with yolk. Their nuclei may afterwards retreat 

 outwards and form a new epithelium (Astacus, &c.) which 

 encloses the yolk. Not only may the invagination for the 

 entoderm disappear altogether, but when it is retained 

 it may take the most unexpected forms. In Chironomus 

 and other insects it is on general grounds .likely that 

 the tissue formed by infolding is really the entoderm, 

 from which the mesoderm is afterwards differentiated. 

 The details still require to be elucidated by practised 

 embryologists. 



Position of At this time (end of first day of hatching) the parts of 



embryo at • i r- m • • i • m ^(^^ 



end of first the embryo are m the lollowmg position (ng. 122;: — Ine 

 '^^^' body is curled up within the egg, l3^ing in the median 



longitudinal plane, with its ventral surface close to the 

 egg-shell, and the dorsal surface, which is largely open, 

 in contact with the yolk. The head is thrown back and 

 lies on the dorsal surface. The tail-end is at a short 

 distance, and between the two is a thin sheet of extra- 

 embr37onic blastoderm. At this point the yolk projects 

 between the head and tail, which are therefore distinctly 

 marked out. 

 Envelopes The edgcs of the ventral plate pass into the extra- 

 embryonic blastoderm, which retains its original character 

 of a single layer of cells. On the sides of the future body 

 this tract will gradually be encroached upon by the 

 extension of the ventral plate, which grows upwards on 

 either side, and ultimately completes the body-wall ; 

 between the head and tail, temporarily in apposition, the 



' Metschnikoff, Zciisch.f. wiss. Zool, Bde. xxiv-v ,1874-5). 



