262 SPERMATOGENESIS AND OOGENESIS LESS, xxm 



duction is effected either asexually by the fission of the en- 

 tire individual, or in the case of sexual reproduction, follows 

 after two entire individuals have undergone conjugation. In 

 multicellular forms, on the other hand, single cells are set 

 apart for sexual reproduction. 



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. 246), 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 multicellular 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 

 cavity which finally communicates with the exterior by a 

 mouth. 



It is an interesting circumstance that these embryonic 

 stages are to some extent paralleled by certain adult 

 organisms, two of the more accessible and well-known of 

 which will now be described. 



Pandorina (Fig. 63, A) is a colony consisting of sixteen 

 unicellular zooids closely packed in a gelatinous case of a 

 globular form. Each zooid resembles in general characters 

 a motile Haematoccus or Euglena, having an ovoid cell-body 

 coloured green by chlorophyll, a red pigment-spot, and 

 two flagella, which protrude through the gelatinous wall of 

 the colony, and by their action impart to it a rotatory 

 movement. 



In asexual reproduction each of the sixteen zooids divides 

 and re-divides, forming at last a group of sixteen cells. In 



