88 MORPHOLOGY. 



At this point we may conveniently, though somewhat arbitrarily, designate tlie developing 

 body as the " embryo." We next find that one end of the oval embryo begins to be prolonged 

 beyond the rest, upon which it becomes bent back as it continues to elongate itself (I). By this 

 time the embryo has become endowed with evident contractility, as manifested by sluggish 

 changes of contour. 



Shortly after this the embryo escapes from its confinement by the rupture of the walls of the 

 gonophore, when it speedily straightens itself, and then, in the form of an elongated, nearly 

 cylindrical body, slightly tapering towards one end, is discharged through the summit of the 

 gonangium into the surrounding water (K). 



We now find that its whole surface is clothed with vibratile cilia, by whose aid it moves 

 slowly along the bottom of the vessel, while the cells and granules which occupied its deepest 

 parts seem to have undegone a kind of liquefaction, resulting in the formation of an elongated 

 cavity in the axis of the embryo which is thus, at this period, a nearly cylindrical sac, without, 

 as yet, any appearance of a mouth, but with an endoderm and ectoderm already differentiated, 

 while multitudes of very minute elongated oval bodies, with a high refractive power, soon make 

 their appearance in the ectoderm ; these are most pi'obably thread-cells, though no sign of a 

 filament can as yet be discovered in them. 



I am unable to form any decided conclusion as to whether the endoderm, which thus about this 

 period becomes demonstrable, is to be regarded as the remains of the more central cells of the 

 segmented ovum, or as an inner lamina formed by a differential splitting of the peripheral layer 

 or blastoderm. Tlie appearances are rather in favour of the former view ; but if the latter be the 

 true interpretation, the analogy up to a certain point with the development of the vertebrate ovum 

 will be the closer. At all events there can be little doubt that the two membranes which now 

 make their appearance and continue as the endoderm and ectoderm of the developing hydroid 

 are fimctionally equivalent, the endoderm to the internal or vegetative layer, and the ectoderm 

 to the external or animal layer of the vertebrate blastoderm. It will be seen in the sequel that 

 the parts which are concerned in digestion and in generation have their seat in the endoderm, 

 while those which are destined for the fimctions of sensation, locomotion, and protection, originate 

 in the ectoderm — a state of things which has its exact parallel in the two laminae of the vertebrate 

 blastoderm.^ 



We have thus arrived at the ciliated and locomotive stage of the embryo. To this stage Sir 

 .John G. Dalyell has given the name of " planula" — a name, however, suggested by a mistaken 

 view of its form, which he compares to a Phinaria. In this comparison he has probably 

 been led astray liy the imperfection of his microscope ; for the locomotive embryo has no 

 tendency whatever to a flattened shape, as indicated by the name of " planula," but is always 

 conical or cylindrical. Instead of " planula," therefore, one is strongly tempted to employ for 



^ The comparison of the structure of the Hydrozoa to the early stages in the development of 

 the highest animals has been very distinctly made by Professor Huxley. " The outer and inner 

 membranes appear to bear the same physiological relation to one another as do the serous and mucous 

 layers of the germ ; tlie outer becoming developed into the muscular system and giving rise to the 

 organs of offence and defence ; the inner, on the other hand, appearing to be more closely subservient 

 to the purposes of nutrition and generation." — " On the Anatomy and Affinities of the Medusce." 

 'Phil. Trans.,' 1849, p. 426. 



