342 ANIMAL DEVELOPMENT 



but the following sketch of the development of a simple typical 

 sponge may not be without interest (fig. 862). The fertilized egg- 

 cell divides repeatedly to give rise to a solid Blastula {inorula), the 

 cells of which next arrange themselves into two concentric layers 

 {ectoderm and endoderm), while a central digestive cavity also 

 makes its appearance, very much as in Hydra. The outer cells 

 develop a covering of cilia, by means of which the embryo, or 

 rather larva, which is now of oval form, swims about for a time. 

 The term larva has elsewhere been explained, but it may not be 

 superfluous to repeat here that it is applied to an embryo which 

 leads an independent existence, and differs more or less markedly 

 from the adult. This particular sort of larva is known as a 

 Planula, and is also a stage in the life-histories of many Ccelen- 

 terates. It is at first mouthless, but a mouth is soon developed, 

 and a typical gastrula stage is thus attained. The larva now fixes 

 itself by the mouth end, and loses its covering of cilia. A new 

 opening makes its appearance at the free extremity, and becomes 

 the osculum of the adult. Further important changes consist in the 

 appearance of a large number of small holes in the body-wall, and 

 the development of a layer {inesoglcea) between ectoderm and en- 

 doderm. Cells pass into this, and some of them give rise to three- 

 rayed spicules of calcareous nature, each ray taking origin from a 

 separate cell; others become egg-cells. By subsequent budding 

 a colony may be formed. 



Early Stages in the Development of the La-iicelet iyAmphioxus). 

 — While Coelenterates and Sponges essentially consist of only two 

 continuous sheets of cells {ectoderm and endoderm), although a 

 third jelly-like layer {inesoglcea)) afterwards makes its appearance 

 between these, and usually contains cells of various kind, a further 

 stage is reached in higher groups of animals. For in any one of 

 these the embryo, after passing through a two-layered or gastrula 

 stage, develops a third sheet of cells {inesodernt) between ectoderm 

 and endoderm_. Many difficult problems centre in the mesoderm, 

 but with these we are not here concerned. 



The three-layered animals, of which the Lancelet is selected 

 as a type, illustrate the principle of division of physiological labour 

 in a much higher degree than do the two-layered forms, and are 

 consequently of more complex structure. This has already been 

 abundantly illustrated in preceding sections, and will become still 

 more obvious when we have to consider the Nervous System and 



