rst large publication, the _Beitraege zur Geschichte
der Thierwelt_ (i.-iv., Halle, 1820-27), contained much anatomical work
in addition to the purely embryological; he commenced here his series of
papers on the development of the genital and urinary organs, continued
in the _Abhandlungen zur Bildungsund Entwickelungs-Geschichte des
Menschen und der Thiere_ (i., ii., Leipzig, 1832-3). A fellow-worker in
this line was Johannes Mueller, whose _Bildungsgeschichte der Genitalien_
(Duesseldorf) appeared in 1830.

In a memoir on the development of the crayfish which appeared in
1829,[192] Rathke found in an Invertebrate confirmation of the germ-layer
theory propounded by Pander and von Baer. He was greatly struck by the
inverted position of the embryo with respect to the yolk. In following
out the development of the appendages he noticed how much alike were
jaws and legs in their earliest stage, and how this supported Savigny's
contention that the limbs of Arthropods belonged to one single type of
structure. In his paper (1832) on the development of the fresh-water
Isopod, _Asellus_,[193] Rathke returns to this point. Commenting on the
original similarity in development of antennae, jaws and legs, he writes,
"Whatever the doubts one may have reserved as to the intimate relation
existing between the jaws and feet of articulate animals after the
researches of Savigny on this subject and mine on developing crayfish,
they must all fall to the ground when one examines with care the
development of the fresh-water Asellus" (p. 147 of French translation).

Further comparative work by Rathke is found in the two volumes of
_Abhandlungen_ and in a book, _Zur Morphologie, Reisebemerkungen aus
Taurien_ (1837), which contains embryological studies of many different
types, including a study of the uniform plan of arthropod limbs. Later
on Rathke devoted himself more to vertebrate embryology, producing among
other works his classical papers on the development of the adder (1839),
of the tortoise (1848), and of the crocodile (1866). He laid the
foundations of all subsequent knowledge of the development of the
blood-vascular system in a series of papers of various dates from 1838
to 1856. The diagrams in his paper on the aortic arches of reptiles
(1856) were for long copied in every text-book.

Rathke was a foremost worker in another important line of embryological
work, the study of the development of the skeleton and particularly of
the sk