260 SPERMATOGENESIS AND OOGENESIS 



In concluding the present lesson, we shall consider 

 briefly a point which has probably already struck the reader. 

 Among the plant-forms which have come under our notice 

 there has been a very complete series of gradations from the 

 simple cell, through the branched cell, linear aggregate, and 

 superficial aggregate, to the solid aggregate, whilst among 

 the animals already discussed there has so far been no 

 attempt to fill up the very considerable gap between the 

 unicellular Infusoria and Hydra, which is not only a solid 

 aggregate, but has its cells arranged in two definite layers 

 enclosing a digestive cavity. 



When we say that no attempt has been made to fill up 

 this gap, we mean as far as adult forms are concerned. If 

 the reader will turn to the account, in the previous lesson, of 

 the development of hydroid polypes (p. 245), he will see that 

 the facts there described do as a matter of fact help 

 us to see a possible connection between unicellular 

 animals and multi cellular two-layered forms with mouth 

 and digestive cavity. The oosperm of the hydroid (Fig. 

 57, A) has the essential character of an Amoeba, the 

 polyplast (E) is practically a colony of Amoebae, and the 

 planula (H) a similar colony in which the zooids (cells) are 

 dimorphic, being arranged in two layers with a central 

 digestive cavity which finally communicates with the exterior 

 by a mouth. In hydroids the mouth is not formed until 

 after the appearance of the tentacles, but in a large propor- 

 tion of the higher animals the polyplast-stage is succeeded 

 not by a mouthless planula but by a two-layered embryo 

 with a mouth at one end, called a gastrula (Fig. 63). This 

 is a very important stage, since it exhibits in the simplest 

 possible way the essential characteristic of a diploblastic 

 animal a two-layered sac with mouth (13 lp} and stomach 

 (/), the outer layer of cells (Ekt) being protective and 



