xvi THE HISTORY OF ZOOLOGY 659 



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, mesoderm, 

 and endoderm in the Vertebrate embryo, and showed that histo- 

 logical differentiation, or the formation of the permanent tissues 

 from embryonic cells, proceeds hand in hand with morphological 

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

 enunciate what is known as von Baer's law, that development is 

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

 isation 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 investigations 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 fertilisation 

 was discovered, and the controversy between ovulists and sperma- 

 tists 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 Muller, was unable 

 to state positively whether or not the sperms were parasitic animal- 

 cules. But in 1843 Martin Barry observed the union of ovum and 

 sperm in the Rabbit, and three years later Kolliker 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 

 conceived 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 pre- 

 sumably 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. 



About the middle of the century the vertebral theory, freed 



