HOilOLOCJY OE TILE GEEMINAL LAYEES. 117 



lining of the alimentary canal are the first differentiations in the 

 embryo ; and many embryos, the so-called Planulae and Gastrulse, on 

 leaving the egg, have only these two layers and an internal cavity, 

 the archenteron. Then follows the development of the nervous 

 and muscular systems, the latter taking place sometimes contem- 

 poraneously with or after the first appearance of the skeleton, 

 especially in cases in which a germinal streak is developed. The 

 urinary organs and various accessory glands, the blood-vessels and 

 respiratory organs do not appear till later. 



The degree of difference between the offspring on attaining the 

 free condition (i.e., at birth or hatching) and the sexually mature 

 adults, both as regards form and size as well as organization, varies 

 considerably throughout the animal kingdom. 



It is a very striking fact that an embryo provided with a central 

 cavity and a body wall composed of only two layers of cells appears 

 in different groups of animals as a freely moveable larva capable of 

 leading an independent life. Having recognized this fact, it was 

 not a' great step, especially as Huxley* some time ago had compared 

 the two membranes of the body wall of the Medusae (called later 

 by Allman ectoderm and endoderni) with the outer and inner 

 layers of the vertebrate blastoderm (epiblast and hypoblast), to arrive 

 at. the conclusion that there was a similar phylogenetic origin for the 

 similar larvae of very different animal types, and to trace back the 

 origin of organs functionally resembling each other to the same 

 primitive structure. 



It was A. Kowalewskit who, by the results of his numerous 

 researches on the development of the lower animals, first gave this 

 conception the groundwork of fact. He not only proved the occur- 

 rence of a two-layered larva in the development of the Co3lenterata, 

 Echinoderms, Worms, Ascidians, and in Amphioxus amongst Verte- 

 brates, but also on the ground of the great agreement in the later 

 developmental stages of the larvae of Ascidians and Amphioxus 

 and in the mode of origin of equivalent organs in the embryos 

 of Worms, Insects, and Vertebrata, protested against the hitherto 

 universally received view implied in Cuvier's conception of types, 

 that the organs of different types could not be homologous with one 

 another. 



* Th. H. Huxley, " On the Anatomy and Affinities of the family of Medusae." 

 Philosophical Transactions. London, 1849. 



t Cf. A. Kowalewski's various papers in the " Memoires de 1'Acad. de Peters- 

 bourg," on Ctenophora, Phoronis, Holothurians, Ascidians, and Amphioxus, 1866 

 and 1867. 



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