INDUSTRIAL PROGRESS DURING THE YEAR 1873. Ixxxix 



thinks that sex is determined by differences in the amount 

 of food, and states her belief that by starving caterpillars, 

 males are the result. The editors of the same journal, and 

 also Mr. Riley, state the well-known fact that the sex of in- 

 sects is known to be determined when or very soon after the 

 insect leaves the egg, and that the sexes are probably deter- 

 mined at the time of conception. 



The subject of embryology, always of great interest to the 

 philosophical naturalist, has acquired a new value in the 

 light of the theory of descent. The essential point of in- 

 quiry now engaging the attention of embryologists, and for 

 which we are especially indebted to the Kussians and Ger- 

 mans, is as to the primary number of germinal layers of cells 

 in the embryo, and whether the germs of all animals are 

 alike at first. The extreme difficulty of these studies may 

 be imagined. By making transverse sections of eggs hard- 

 ened by alcohol and various chemical substances, and by 

 the use of different chemical reagents, we have, however, 

 been able to advance in a wonderful degree the study of the 

 earliest changes in the germ of the articulates, moUusca, and 

 especially the vertebrates, while the minute eggs and em- 

 bryos of the lower animals afford by their transparency op- 

 tical sections. Of almost startlino; interest are the results of 

 Miklucho Macleay and Haeckel's studies on the embryology 

 of the sponges. They show that the germs resemble the 

 young of certain radiates, and are made up of two layers of 

 cells. This, while proving that the sponges are animals com- 

 parable in structure with certain low radiate animals, such 

 as the hydra (they even regard them as homologous in struct- 

 ure with the radiates), shows that the view of Carter, Lie- 

 berkiihn, and James Clark, that they are compound infusoria, 

 is no longer tenable. In fact, Haeckel insists that the sponges 

 are not Protozoa. But it also seems that the " gastrula," or 

 free swimming germ of the sponge, is homologous Avith the 

 " planula," or free swimming germ of the acalcphs. Haeckel, 

 from the closeness of the homoloo-ies which he claims to exist 

 between the adult sponges and acalephs, places them together 

 in the zoophytes. He claims that the famous "germ-layer" 

 theory which Huxley, in 1849, and afterward the distinguish- 

 ed Russian embryologist, Kowalevsky, applied to the inver- 

 tebrates as well as vertebrates, also applies to the sponges. 



