EARLY CHANGES IN THE OVUM. 235 



directly from the germinal layers of the blastoderm ; lately, 

 however, most embryologists have adopted the view which 

 originated with His, that the blood and the connective tissues 

 — in insects the connective reticulum and the fat bodies — arise 

 from amoebiform cells (mesamoeboids), called by His para- 

 blastic, and by the brothers Hertwig mesenchymatous cells. 

 According to R. and O. Hertwig [119] 'very little is known of 

 the origin of the mesenchyme ; it can only be said that in 

 different classes of animals it originates from different sources, 

 and appears at various stages of development, but once 

 formed, it penetrates between the epithelial laminae of the 

 blastoderm and the tissues arising from them, investing, 

 uniting and supporting them.' It consists of cells arranged 

 in no definite order, and separated from each other b}- a 

 copious intercellular matrix.' 



His, as long ago as 1S65 [116], was led to regard the blood 

 and connective tissues of the chick as developed from cells 

 which originate in the great food-yelk, which he believed to 

 wander in between the layers of the blastoderm. These in- 

 wandering cells he termed parablasts. I have frequently 

 m3'self seen numerous amoeboid cells in the yelk of a bird's Qg^ 

 in the early stages of incubation. The above view of His met 

 with little acceptance for a long time., and he subsequently 



Bibliography. — Those who are interested in following up the views of 

 His and Hertwig further should consult the following works : 



116. His, \V., 'Die Hiiute und Hohlen des Korpers.' Basle, 1865. 



117. His, W., ' Der Keimwalldes Hiinereiesund die Entstehung der Para- 

 blastischen Zellen.' Zeits. f. Anat. und Entwickelungsgeschichte, 1876. 



118. His, W., ' Unsere Korperform und das physiologische Problem ihrer 

 Entstehung. Briefe an einen befreundeten Naturforscher.' Leipzig, 



1879. 



119. Hertwig, O. and R., ' Die Coelomtheorie. Versuch einer ErkUirung 

 des mittleren Keimblattes.' Jena, iSSi. 8vo. Studien zur Blatter- 

 theorie. Heft iv. 



120. His, \V., ' Die Lehre von Bindesubstanzkeim (Parab)ast). Riickblick 

 nebst kritischer Besprechung einiger neuerer entwickelungsgeschicht- 

 licher Arbeiten.' Archiv f. Anat. und Phy. Anat. Abth., 18S2. 



121. WiELOwiEjSKi, H., ' Ueber das Blutgewebe der Insecten.' Zeitsch. 

 f. w. Zool., Bd. xliii., 1886. 



