26 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



reception of external stimuli, in this case chemical. They are 

 turned in those directions whence food is most likely to approach, 

 i.e., the oral end of body and tips of tentacles. The external sur- 

 faces of the large ectoderm-cells appear to receive those stimuli 

 which lead to movements. 



Spontaneity is best shown by the co-ordination of the cells for 

 the performance of useful movements, such as those of locomotion, 

 which cannot all be directly due to external influences. The co- 

 ordinative power certainly resides in the ectoderm, either in the 

 large cells or nerve-cells, or probably in both. The discharge of 

 the nematocysts also seems to depend on the "will" of the 

 animal. If a cnidocil is touched, an impulse of some sort appears 

 to be transmitted to the connected nerve-cell, which, in the case 

 of food, sends back an impulse leading to the eversion of the cell, 

 while a particle of sand, say, causes no such disturbance. 



DEVELOPMENT. 



In each ovary, as a general rule, only one ovum is developed, 

 which when almost mature projects freely to the exterior, as a 

 result of the rupture of the cells which cover it. Before fertil- 

 ization the ovum divides into two unequal parts, the smaller of 

 which is known as a polar cell. A second polar cell is next 

 formed by further subdivision, and the nucleus of the ovum is 

 now known as the female pronucleus. The two polar cells perish 

 without taking any share in the further development. 



Fertilization now ensues, a single sperm fusing with the ovum, 

 and its nucleus (male pronucleus) uniting with the female pro- 

 nucleus to constitute the nucleus of the oosperm (segmentation 

 nucleus). The oosperm at once commences to develop, and 

 during the earlier stages remains attached to the parent. The 

 first process is that of segmentation or cleavage, consisting of a 

 series of cell-divisions by which the unicellular oosperm is con- 

 verted into a cellular mass. It is, in fact, a case of continued 

 fission, but the products of division, instead of separating and 

 becoming distinct organisms, as in Vorticella and the like, remain 

 connected together and constitute the rudiment of a multicellular 

 animal. The cleavage in Hydra consists of a series of bipartitions, 

 so that the embryo consists of 2, 4, 8, 16, 32, &c., cells succes- 

 sively. Cleavage is here said to be both complete (holoblastic) 



