100 



PRINCIPLES OF EMBRYOLOGY 



before fertilisation till the time of the second or third cleavage) a locaHsa- 

 tion of types of cytoplasm which control the direction of later develop- 

 ment; but one of these substances, namely that which normally gets into 

 the D quadrant, can in some species cause a considerable amount of re- 

 organisation to go on in its neighbourhood, so that in eggs of this type a 

 whole embryo tends to be formed around it. 



Figure 6.4 



The polar lobe in the mollusc Dentalium. In the uncleaved egg {A) there are 

 specialised ooplasms at both animal and vegetable pole. At the first division 

 (J5), the vegetative pole plasm is protruded as a 'first polar lobe' (Sj). After 

 the division this is withdrawn into the CD blastomere, whence it protrudes 

 again as a 'second polar lobe' (Sg) at the second division D, after which it is 

 withdrawn once more into the D blastomere. At E is shown the normal 

 trochophore larva ; at F and G the defective larvae produced when the first 

 (F) or the second (G) polar lobe is removed. (After Wilson.) 



Recent investigations have tended to concentrate on two problems; 

 to discover more about the nature of the substances whose localisations 

 are responsible for the mosaic aspects of development in these forms, and 

 to try to find out the factors which bring about the localisation. Some 

 progress has been made in both directions. 



Local differences in the cytoplasm, which are invisible in normal life, 

 can sometimes be revealed by treatment with suitable dyes or histo- 

 chemical reagents. Vital dyes which act as pH or rH indicators (such as 



