i68 AN INTRODUCTION TO ZOOLOGY 



a medusa is being formed, and then on into the medusa. Such a 

 migration of the germ cells has been observed in a number of other 

 invertebrates, and also in certain vertebrates, in all of which cases 

 the primordial germ cells are first recognisable at some distance 

 from the points at which the gonads will later be formed. Further- 

 more, in some cases a mother sexual cell is to be distinguished at a 

 very early stage in the cleavage when only a few cells are present. 

 From these facts we may draw a line of demarcation between the 

 ordinary cells that go to form the body, the somatic cells, and the 

 cells destined to give rise to all the germ cells, and we also see that 

 the latter are to a certain degree independent of the former, a con- 

 sideration that is often referred to in discussing theories of heredity. 

 The medusae float about in shoals of enormous numbers, 

 and when the germ cells are ripe the gonads rupture, setting the ova 

 and spermatozoa free in the sea where fertilisation occurs. Thus 

 the medusa serves not only for multiplication but also for the 

 dissemination of the species. The zygote undergoes complete and 

 regular segmentation, leading, as in Hydra, to the formation of a 

 hollow sphere of cells, the blast ula. The cavity within it, the blasto- 

 ccel, is later filled in a similar way by cells derived from the periphery, 

 but these cells are only produced at one part of the circumference, 

 and consequently the process is described as unipolar delamination 

 and immigration. Thus arises a solid embryo with an outer layer 

 of columnar cells, the ectoderm, enclosing a solid mass of entoderm 

 cells. This elongates, develops a coating of cilia, and leads an 

 independent life, floating near the surface of the sea. When, in an 

 early stage of its development, before it resembles the adult, an 

 animal lives freely and independently we term it a larva, and in the 

 particular case of Obelia the larval form is known as a Planula. It 

 is quite a common stage in the class Hydrozoa, although missing in 

 Hydra itself. Sooner or later the entoderm cells become arranged 

 around an internal cavity, the enteron, which makes its appearance 

 as a slit. The larva then settles down and attaches itself by an 

 expanded basal disc to some object in the sea, often, in the case of 

 0. geniculata, a frond of Laminaria. It sheds its coat of cilia and 

 becomes transformed into a hydroid individual by the enlargement 

 of the distal extremity and the appearance of tentacles, hypostome 

 and mouth. The ectoderm now secretes a tubular investment of 

 perisarc, and in this condition, when it is a simple hydra-like animal 

 with a marked basal or attachment disc, it is termed a hydrula. 

 It starts now to grow into a colony : a bud appears some way up the 

 stalk, and this turns into a second hydranth, soon to give off a third 

 polyp, and so we have the hydrocaulus formed. Simultaneously with 

 this, small processes arise from the attachment disc that grow out and 



