PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 



115 



of fertilized eggs. Asexual and sexual repro- 

 duction may both occur at the same time in 

 an individual. 



Budding (Fig. 50) 



Asexual (sexless) reproduction by a proc- 

 ess of budding is a common occurrence in 

 the hydra. Several buds are often found on 

 a single animal. Superficially the bud first 

 appears as a slight bulge in the body wall. 

 This pushes out rapidly as a projection which 

 soon develops a circlet of blunt tentacles 

 about its outer end. The cavities of both 

 stalk and tentacles are at all times directly 

 connected with that of the parent. When 

 full grown, the bud becomes detached and 

 leads a separate existence; this requires about 

 two days when conditions are favorable. 

 Budding may occur at almost any season. 



Sexual reproduction 



Both ova and spermatozoa appear to de- 

 velop from interstitial cells. Some species of 

 the hydra form both sperms and eggs in one 

 individual, but in others only one sex oc- 

 curs. There may be as many as 20 or 30 

 testes; each is a conical outgrowth. The sex- 

 ual state can be induced in some species by 

 lowering the temperature; this accounts for 

 the appearance in Hydra oligactus of sex 

 organs in the autumn and during early win- 

 ter. 



Spermatogenesis. The male germ cells of 

 the hydra are formed in little conical or 

 rounded elevations called testes, which pro- 

 ject from the surface of the body (Fig. 51). 

 An indefinite number of interstitial cells 

 collect locally into a mass, causing the epi- 

 dermis of the animal to bulge. Each of these 

 interstitial cells is a primordial germ cell; 

 it gives rise by mitosis to a variable number 

 of spermatogonia; these divide to form pri- 

 mary spermatocytes, which give rise by divi- 

 sion to secondary spermatocytes; these 

 divide, producing spermatids which trans- 

 form into spermatozoa. The mature sper- 

 matozoa swim about in the distal end of the 

 testis and finally escape to the exterior 



through one or more small fissures in the 

 protective covering. In most hydras definite 

 nipples are formed on the testes, through 

 which the sperms escape (Fig. 50). The 

 mature spermatozoa swim about in the 

 water searching for an egg. 



Oogenesis. The egg is an interstitial cell 

 which becomes large and spherical and pos- 

 sesses a large nucleus (Fig. 51). Several in- 

 terstitial cells begin to enlarge to form 

 ovocytes but one finally incorporates the 

 others. As the ovum grows it becomes scal- 

 lop-shaped, due to confinement between the 

 columns of the supporting cells. When fi- 

 nally it attains full growth, it becomes 

 spherical; but it is still surrounded by epi- 

 dermal cells, which stretch enormously to 

 cover the egg and still remain rooted to the 

 mesoglea. (Illustrations showing a layer of 

 epithelial cells covering the egg, but separate 

 from the mesoglea, are incorrect; although 

 such false interpretations in sections are easy 

 to make.) Maturation now takes place. Two 

 polar bodies are formed, the first being 

 larger than the second. During matura- 

 tion the number of chromosomes is re- 

 duced from the somatic number 12 to 6; 

 this occurs at the end of the growth period. 

 Now an opening appears in the epidermis 

 and the egg is forced out, becoming free on 

 all sides except where it is attached to the 

 parent. 



Fertilization. Fertilization usually occurs 

 about as soon as the egg is extruded. Several 

 sperms may penetrate the egg membrane, 

 but only one enters the egg itself. The sperm 

 brings a nucleus containing 6 chromosom.es 

 into the egg. The male and female nuclei 

 unite, forming the fusion nucleus. 



Embryology. The cleavage, which now be- 

 gins, is total and regular. A well-defined 

 cleavage cavity is present at the end of the 

 third cleavage, the eight-cell stage. V.^hen 

 the blastula is completed, it resembles a 

 hollow sphere with a single layer of epithelial 

 cells composing its wall (Fig. 56). These 

 cells may be called the primitive ectoderm. 

 By mitotic division they form endoderm cells 



