REPRODUCTION 531 



celled green forms living today that show brought in proximity to each other. How- 

 a graded sequence from the union of cells ever, when animals invaded the land the 

 that are similar in size and activity to those whole process became much more complex, 

 that are quite unlike in these respects ( Fig. It must have taken a long time to accom- 

 21-1). At certain times, individual cells of plish this transition along with the many 

 Chlamijdomonas will fuse, forming a zygote otliers resulting from the pronounced 

 which will overwinter and continue its change of habitat. Perhaps the most inter- 

 asexual metliod of reproduction the next esting modifications came about among the 

 spring. Since these uniting cells are equal vertebrates, some of which we have already 

 in size, they are called isogametes and their discussed. 



union, isogamy. A closely related form, 



rT7 1 • 1 ui, ^u Care of the young 



Ulva, undergoes a similar process, altnougri 



in this case the cells are unlike in size. The Among the vertebrates there are three 



smaller of the two is more active than the ways in which young are cared for in their 



larger and in this respect resembles a early development (Fig. 21-2). Some are 



sperm. Such unlike gametes are called hatched from eggs that are laid, as in the 



anisogametes and their union, anisogamy. case of most fishes, amphibia, many reptiles 



Among the ciliate Protozoa, such as para- (Fig. 21-3) and all birds. These are called 



mecium, the migrating nucleus during con- oviparous forms. Others retain the eggs 



jugation (see p. 119) is smaller than the within the uterus until they hatch, and the 



immotile one that remains behind. Here resulting young are therefore born in a rela- 



there is not only a size difference but also tively advanced and active stage of de- 



a physiological difference — one moves, the velopment. These are said to be ovovivipar- 



other does not. Once the sex cells reached ous. Some fish and some reptiles (snakes) 



this relationship in their evolution, they are of this type. Still other vertebrates 



then maintained it throughout all higher ( mammals ) produce small eggs without yolk 



forms. The smaller sperm cell is always that develop in the uterus and the young 



motile and is able to maintain sustained receive most, if not all, of their nourishment 



movement, while the egg is large and im- from the uterine wall of the mother. These 



motile. Such gametes are referred to as are said to be viviparous. Young born thus 



heterogametes and their fusion, heter- are, of course, more or less advanced in de- 



ogamy. After sexual reproduction became velopment. 



established it was retained, and while we In general, fishes and amphibians give 



see a rather wide range in sizes and shapes their young little or no care whatever and 



of both eggs and sperms, the fundamental consequently no provisions in the way of 



plan remains unchanged in all animal accessory structures are found in these ani- 



groups. mals (Fig. 21-4). However, when the rep- 



The methods of bringing eggs and sperms tiles moved onto land, certain anatomical 



together is relatively simple among both the modifications were essential if tlie young 



lower invertebrates and the lower verte- were to survive in a dry environment. For 



brates. The union is purely fortuitous, al- one thing, the egg became very large, 



though some arrangement, such as seasonal abundandy supplied with reserve food for 



aggregations, is usually provided so that the developing embryo, thus providing a 



the animals will be in the immediate vicin- means for the embryo to reach a rather ad- 



ity of one another. As long as the animals vanced stage before it had to shift for itself, 



remain in a fluid environment, all that is Besides this food reserve, the egg had to 



necessary is to discharge the sex cells into supply a fluid environment in which the 



the water where by sheer chance they are embryo could develop. In otlier words, a 



