332 E. W. MACBEIDE. 



closely applied to the uterine wall, from which nourishment is 

 obtained, and the yolk having consequently in large measure 

 disappeared. Thirdly — a much rarer case, — a number of eggs 

 are enclosed together in a capsule and only one develops ; the 

 eggs destined to be eaten being known as yolk cells. As a less 

 extreme case we have the eggs all developing up to a certain 

 stage, but only a few surviving. This condition is seen in 

 Prosobranch Mollusca. Lastly, we may mention those cases 

 in which the uterus or other brood-pouch of the mother is 

 used, so to speak, as a nursery for the larvae, the embryos 

 escaping from the egg-membrane, and passing the earlier part 

 of their existence as free-swimming organisms inside the 

 brood-pouch. 



Taking the first case, which is by far the commonest, the 

 disturbances of development which are found in it are due to 

 two main causes — yolk and the egg-membrane. It is owing 

 to the cramping influence of the latter that external differentia- 

 tion of- form is to a large extent lost. The gastrula of 

 Asterina gibbosa, for instance, is almost spherical, con- 

 trasting thus with the common form of Echinoderm gastrula, 

 which is more or less elongated. Where the egg is enclosed in 

 a roomy capsule, on the other hand, as in the Pulmonata, this 

 is less frequently the case ; for instance, we have the velum of 

 Limnseus and Planorbis. Mere disuse will not suffice to 

 account for the disappearance of external organs, as in this 

 case all traces of ancestral history ought to disappear in internal 

 as well as external organs, and this is not the case. 



The presence of food yolk exercises the most distorting 

 influence on development. To Lankester^ is due the credit of 

 first laying emphasis on this. In treating of the development of 

 Mollusca he points out that the question whether the endoderm 

 is represented by many or few cells, and whether, consequently, 

 these are invaginated to form the gut, or whether the ectoderm 

 grows over them, is entirely determined by tlie amount of yolk 

 present. Balfour, who had almost at the same time instituted a 



1 " On the Invaginate Planula or Diploblastic Phase of Paludina vivi- 

 para," E. Ray Lankester, ' Quart. Journ. Micr. Sci.,' vol. xv, 1875. 



