(237) C. B. Reichert. "Beschreibung einer frühzeitigen menschlichen Frucht im bläschenformigen Bildungszustande (sackformiger Keim von Baer) nebst vergleichenden Untersuchungen über die bläschenformigen Früchte der Säugethiere und des Menschen." _Abhandlungen der königl. Akad. d. Wiss. zu Berlin_, 1873. (238) Allen Thomson. "Contributions to the history of the structure of the human ovum and embryo before the third week after conception; with a description of some early ova." _Edinburgh Med. Surg. Journal_, Vol. LII. 1839. CHAPTER XI. COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS AND OF THE EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES. Although the preceding chapters of this volume contain a fairly detailed account of the early developmental stages of different groups of the Chordata, it will nevertheless be advantageous to give at this place a short comparative review of the whole subject. In this review only the most important points will be dwelt upon, and the reader is referred for the details of the processes to the sections on the development of the individual groups. The subject may conveniently be treated under three heads. (1) The formation of the gastrula and behaviour of the blastopore: together with the origin of the hypoblast. (2) The mesoblast and notochord. (3) The epiblast. At the close of the chapter is a short summary of the organs derived from the several layers, together with some remarks on the growth in length of the vertebrate embryo, and some suggestions as to the origin of the allantois and amnion. Formation of the gastrula. Amphioxus is the type in which the developmental phenomena are least interfered with by the presence of food-yolk. In this form the segmentation results in a uniform, or nearly uniform, blastosphere, one wall of which soon becomes thickened and invaginated, giving rise to the hypoblast; while the larva takes the form of a gastrula, with an archenteric cavity opening by a blastopore. The blastopore rapidly narrows, while the embryo assumes an elongated cylindrical form with the blastopore at its hinder extremity (fig. 169 A). The blastopore now passes to the dorsal surface, and by the flattening of this surface a medullary plate is formed extending forwards from the blastopore (fig. 169 B). On the formation of the medullary groove and its conversion into a canal, the blastopore opens into this canal, and gives rise to a neurenteric passage, leading from the neural canal into the alimentary tract (fig. 169 C and E). At a later period this canal closes, and the neural and alimentary canals become separated. [FIG. 169. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.) The parts in black with white lines are epiblastic; the shaded parts are hypoblastic.