130 



PRINCIPLES OF EMBRYOLOGY 



ectoderm, in Chrysopa and Leptinotarsa. They both found, however, in 

 contradistinction to Krause, that the ectoderm could self-differentiate 

 histologically from an early stage quite independently of the presence of 

 mesoderm, although in Chrysopa its morphogenesis is abnormal if it is not 

 underlaid by mesoderm. Haget in particular has studied the gradual acqui- 

 sition by the various regions of the ectoderm of the capacity for self- 

 differentiation, and the accompanying loss of its abihty to regulate. He 

 has shown that the process is dependent on an influence which spreads 

 from the differentiation centre through the sheet of ectoderm; he speaks 

 of it as a process of 'intra-dermal induction', and it may be compared with 

 the 'individuation' or 'regionalisation' by which various organs become 

 localised within the sheet of invaginated mesoderm in the vertebrates (Fig. 

 8.7). 



p.end 



FlGXJM 8.7 



Results of cauterisation of the endo-mesoderm or ectoderm in Leptinotarsa. 

 Figure i shows the whole median plate [pl.tn.) of the germ-band destroyed. 

 In the resulting embryo (Figure 2) the ectodermal organs are present {tr. 

 tracheae, g. ganglion, app. appendages) but there are no mesodermal or 

 endodermal tissues. In Figure 3 one of the lateral plates of the germ-band 

 (presumptive ectoderm) has been cauterised. The embryo formed (Figure 

 4) lacks ectodermal organs in the cauterised region, and the endodermal and 

 mesodermal organs are also absent where the ectoderm is defective. Figure 

 5 shows a section through a normal embryo {p. end., endodermal lining of 

 gut, m., muscle, im., midgut, tmi., mesodermal tunic of midgut). (After 



Haget I953-) 



