680 ZOOLOGY sect. 



that Amoebae and other lowly organisms were formed entirely of 

 protoplasm, or, as he called it, sarcode. These discoveries paved 

 the way for the generalisations of Max Schultze and De Bary, 

 that the essential constituent of the cell is protoplasm, and that 

 the protoplasm of animals and plants is identical. 



In embryology, the most important work of this time was that of 

 K. E. von Baer, who, in 1827, discovered the ovum of Mammals. 

 He also described three primary germ-layers — ectoderm, meso- 

 derm, and endoderm — in the Vertebrate embryo, and showed that 

 histological differentiation, or the formation of the permanent 

 tissues from embryonic cells, proceeds hand in hand with morpho- 

 logical differentiation or the evolution of organs. He was thus led 

 to enunciate what is known as von Baers law, that development is 

 a progress from the general to the special, and to frame the 

 generalisation that embryos of animals belonging to various 

 classes closely resemble one another in their earlier stages, but 

 diverge more and more as development proceeds. His investiga- 

 tions led him to support Cuvier's view of the division of the 

 animal kingdom into distinct and clearly separated types or 

 branches. 



It was during this period also that the real meaning of fertilisa- 

 tion was discovered, and the controversy between ovulists and 

 spermatists finally .set at rest. Artificial fertilisation had been 

 tried in the last century, but up to 1842 the greatest physiologist 

 and most accurate anatomist of his time, Johannes MuUer, was 

 unable to state positively whether or not the .sperms were parasitic 

 animalcules. But in 1843 Martin Barry observed the union of 

 ovum and sperm in the Rabbit, and three years later Kblliker 

 proved that the sperms were developed from the cells of the testis. 



The period under consideration also saw the development of a 

 school of .speculative or deductive zoology. In 1790 Goethe con- 

 ceived the idea that the skull of Vertebrates is made of modified 

 vertebrae — in other words, that the skull is the highly differentiated 

 anterior end of the backbone. This theory, which may be taken 

 as a type of morphological speculation in the pre -evolutionary 

 period, was re-enunciated and greatly elaborated in 1807 by 

 Lorenz Oken, whose conclusions are worthy of mention, if only 

 to show the dangers of the deductive method in natural science, 

 and the lengths to which unbridled speculation may carry a 

 presumably sane man. He did real service by demonstrating the 

 secondary segmentations of the bony skull ; the occipital segment 

 being his " ear vertebrae," the parietal his "jaw vertebrae," and the 

 frontal his " eye vertebrae." But he clearly went beyond the limits 

 of legitimate .speculation when he contended that the nasal cavity 

 is a cephalic thorax and the mouth a cephalic abdomen ; that the 

 bones of the upper jaw are homologues of the fore-limbs, the 

 lower jaw of the hind-limbs, and the teeth of the digits. 



