HISTORICAL ACCOUNT OF EMBRYOLOGY. 523 



germinal laminae. According to his view, the entire composite system 

 of the intestines is developed from a single lamina. 



His doctrines were enlarged and completed by Pander.* The 

 blastoderm, he maintains, is composed of a layer of granules, which 

 serves as the rudiment of the future inferior germ-layer. A second 

 layer of similar granules develops upon this layer, even during the 

 first hours of incubation, and constitutes the future superior layer, so 

 that the blastoderm at the twelfth hour of incubation consists of two 

 lamiiue, the upper one of which he named the serous, and the lower 

 the mucous layer. Although he alludes also to the middle layer, 

 under the term " vascular lamina," his description of it is deficient in 

 clearness ; for, according to him, it is sometimes an independent struc- 

 ture in which the vessels are developed, but in other instances it is a 

 consequence of the development of the vessels. Nevertheless, he was 

 perfectly aware of the fact that, after twenty-four hours' incubation, 

 three layers, easily separable from one another, are recognizable in the 

 blastoderm. Pander was also the first who endeavoured to sketch out 

 a general plan showing the development of the organism, which he 

 did by referring all the structures and organs that had been termed 

 "animal" from the time of Bichat and Reil to the superior germ- 

 lamina (or epiblast) ; to these belong the nervous system, with the 

 organs of sense ; the muscular and the osseous system. The middle 

 lamina (or mesoblast) he regarded as being simply a vascular lamina ; 

 whilst the lower (endoblast) he considered to contain the rudiments of 

 the intestinal system and its associated glands. 



The researches of v. Baer t corroborated those of Pander, and may 

 be regarded as a continuation of them ; v. Baer described a layer in 

 the non-incubated egg as the first rudiment of a blastoderm ; but, in 

 opposition to Pander, he considered that in the course of the first few 

 hours of incubation a second lamina was formed beneath, not above, 

 this layer, which constituted the rudiment of the endoblast. The 

 development of the mesoblast (vascular lamina) was described in a 

 manner accordant with the account given by Pander, v. Baer con- 

 siderably enlarged our views in respect to the general plan of 

 development. He demonstrated the participation of the mesoblast 

 in the formation 0f the fibrous ludiment of the intestine, as well as in 

 that of the associated glands. He stated that the two layers origi- 

 nating during the first hours of incubation, each divided again into 

 two layers, of which the upper formed the integumental and muscular 



* LOG. cit. 



t Ueber die Entwickelungsgeschichte der Thiere, 1822, i. 



