REPRODUCTION AND HEREDITY 271 



the larvae develop from unfertilized ova which originate in 

 the interior of the sporocyst. As the ova are not generated 

 by an adult form but by the undeveloped larval form, this 

 process is described as psedogenesis, as distinguished from 

 parthenogenesis. 



Before we enter upon the internal phenomena of sexual 

 reproduction, and in particular the structure and genesis of 

 germ-cells, let us cast a brief glance at one of the most attrac- 

 tive and beautiful parts of zoology, the care of the parents 

 for their offspring. 



In the lowest classes of Metazoa, the sponges and jelly-fishes, 

 a proper care of offspring does not exist. As the majority of 

 these forms are fixed to the spot but lead in their earlier stages 

 a free-swimming life, the parents are for this reason alone pre- 

 vented from watching over the well-being of their offspring. 

 But in sponges and most jelly-fishes the ova pass at least the 

 first stages of their development in the maternal body, and leave 

 it only as independent larvae when they are nearly capable to 

 struggle through life without assistance. 



Most unfavourable among all animals is doubtless the condi- 

 tion of the brood of many intestinal parasites whose progenitors 

 remain behind in the interior of their host while the descendants 

 are ejected with the faeces as ova, and handed over defenceless to 

 all the perils of a protracted development. Countless individuals 

 meet with an early death, and the preservation of the species 

 is only secured by the immense fertility of these parasites. 



Many other higher animals, in particular numerous insects* 

 are prevented by their death, which often follows upon egg- 

 laying, to watch over the fate of their young, but they endeavour 

 to compensate them by depositing the germ-cells in safe places 

 or surrounding them with protective covers, and at the same time 

 providing that the young on being hatched shall find sufficient food. 

 We saw a practical illustration of this kind in the gall-flies 

 which deposit their eggs in the interior of plant-parts, grant- 

 ing at once protection and abundant food. Other insects seek 

 out certain animals whom they compel to shelter and feed their 

 brood. Such a case is shown in the illustration. We see here 

 a caterpillar whose entire surface is covered with cocoons of an 

 ichneumon-fly (fig. 63). 



