DISCOVERY OF THE BLASTODERMIC ELEMENTS. tSS 



Tarior.s parts indifferently or equally throuyhout its whole extent, but 

 in the following divisions, viz., First, by a central mesial or axial part, 

 out of which proceed the rudiments of the protovertebral segments of 

 the body ; and, Second, by two lateral parts which undergo subdivision 

 into an upper and lower lamina, the first of these subdivisions contain- 

 ing the rudiments mainly of volunto-motory parts, the walls of the 

 body, or somato-plenral elements ; and the seconct forming the involunto- 

 mofcory parts, as in the walls of the alimentary canal, heart, &c., or 

 splanchno-pJcural elements : the space formed by the separation of these 

 two sets of parts is the visceral or pleuro-peritoneal cavitij. 



From the foregoing enumeration of the several parts of the embryo 

 which are traceable in their origin to one or other of the layers ot 

 the blastoderm, it must not be inferred that these initial elements 

 remain each distinct or separate from the rest, while undergoing 

 the formative changes of conversion. Some of them, doubtless, do 

 maintain their independence in a remarkable degree, as is the case 

 with most of the parts derived from the hypoblast, and some of those 

 fi'om the epiblast ; but in the case of parts proceeding from the meso- 

 blast, this independence is in a great measure lost ; and notwith- 

 standing the original separation, we see, especially in the vascular and 

 nervous systems and in the connective tissue, that in the course of their 

 farther development, there is a great amount of spreading of one into 

 the other sets of the blastodermic elements. 



Discovery of tlie Blastodermic Elements. — We owe to C. F. Wolff, as de- 

 scribed in his Theoria Genera, tionis, published in 1759, the first proof derived from 

 observation of the actual new formation of, the organs of the embryo (epi- 

 genesis) from the simple granular (cellular) elements of the yolk, and to a later 

 work of the same author (On the Formation of the Intestine, which originally 

 appeared in 17(')9,and -s\-as republished in German by J. F. Meckel in 1812) the first 

 suggestion of the laminar constitution of the germ. The full discovery, how- 

 ever, of the three layers of the blastoderm, and especially their relation to the 

 development of the organs and systems of the embrj-o, was, under the influence 

 of DoeUinger's teaching at Wiirzburg, the work of Pander, and was first published 

 in his inaugural dissertation at that University in 1817. The segmentation of 

 the yolk, noticed by Swammerdam and Spallanzani, was first described in 

 batrachia by Prevost and Dumas in 1823, in a Memoir which was followed by 

 an important series of contriljutions by the same authors to the history of the 

 development of reptiles, birds, and mammals. Tlie discovery of the germinal 

 vesicle of the bird's egg by Purkinje in 1825 led the way to more minute obser- 

 vation of the constitution of the germinal part of the ovum. But the founda- 

 tion of embryology as a modem science was most surely laid by C. E. von Eaer 

 of Konigsberg (origin alty. the associate of Pander and pupil of Doeilinger), who 

 discovered the ovum of mammals in 1827, and in his work entitled " Die 

 Entwickelungsgeschichte der Thiere, Beobachtungen und Reflexion en," 1829 — 

 1837, gave the fullest, the most accm-ate, and the most philosoiDh:cal account of 

 the development of animals which has ever appeared. The contemporaneous 

 researches of H. Eathke. also the pupil of Doellinger along with Pander and 

 Von Baer, contributed greatly to the advance of embryological knowledge. 



The investigations of Schwann •' On the conformity in the structure and gro'n'th 

 of plants and animals," published in 1839, threw new light upon the histological 

 composition of the o^■um and blastoderm and tlieir relation to the phenomena 

 of development (see^ General Anatomy, p. 6 et srt/.) ; and in the years contem- 

 poraneous with Yon Baer"s researches, and following their publication, many 

 important contributions appeared which greatly extended the scientific knowledge 

 of the subject ; among the authors of which may be mentioned here, as the 

 most prominent, the names of Yalentin, Rusconi, R. Wagner, Reichert, 



