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PRINCIPLES OF EMBRYOLOGY 



to say, the whole region is behaving as in some sense a unit within which 

 some property, which controls future development, is distributed in a 

 graded manner. When the region is affected by some substance toxic to it 

 (lithium in this instance) it reacts as a whole, every part as it were sinking 

 in grade, so that the more central parts take on the character normally 

 proper to more peripheral regions. 



Again in Linmea, Raven has demonstrated the importance of typical 

 processes of induction. Normally a shell-gland is formed by the ectoderm, 

 in the area where the gut comes into intimate contact with it. Raven was 

 able to induce the blastula to exogastrulate, and showed that if the gut 

 did not touch the ectoderm, no shell-gland was formed. Moreover, in 

 cases of abnormal gastrulation, in which the gut had reached an atypical 

 part of the ectoderm, a shell-gland was formed in this unusual position. 

 Thus there is little doubt that the shell-gland is induced by contact be- 

 tween the gut and the ectoderm. We shall see that such processes play a 

 major role in the development of vertebrates. In the mosaic eggs, al- 

 though there are probably many more of them than have yet been dis- 

 covered, they seem to be of secondary importance in comparison with 

 the process of cytoplasm localisation (Fig. 6.5). 



Figure 6.5 



Induction of the shell gland by the archenteron in Litnnea. 



Normally the shell gland (5.G.) lies posterior to the main band of cilia 

 (Prototroch, Pr), where the archenteron comes in contact with the ecto- 

 derm (i). But if, owing to abnormal gastrulation, the archenteron reaches 

 forward to the pre-trochal ectoderm, the shell gland is formed in the corre- 

 sponding place, (2). (After Raven 1952.) 



SUGGESTED READING 

 Horstadius 1937, Raven 1948, Lehmann 1948a, Costello 1948. 



