16 



PROFESSOR ALLMAN. 



2. The development takes place by a true metamorphosis, 

 the hydroid passing through the larval form of an actinula. 

 Of this Tubularia affords an example. 



3. The development takes place by a true metamorphosis, 

 the hydroid passing through the larval form of a free ciliated 

 (or non-ciliated) planula. Of this we have an example in 

 Sertularia. 



A comparison of the embryonal development of Hydra with 

 that of other hydroids shows that while in some respects 

 important differences are apparent^ yet that in all the more 

 essential features there is a complete agreement, and by 

 widening the comparison so as to embrace in it what we know 

 of the development of the higher animals we arrive at certain 

 comprehensive generalisations which include not only the Hy- 

 droida, but even the highest members of the animal kingdom. 



Huxley was the first to point out the equivalence of the 

 ectoderm and endoderm of the Ccelenterata with the outer and 

 inner germ-lamellce of the vertebrate embryo, and this im- 

 portant generalisation is fully confirmed, not only by the 

 development of hydra, but by that of all other hydroids whose 

 development has been carefully studied. 



There are many groups of the animal kingdom of Avhose 

 developmental history we as yet know little or nothing, but 

 all which have been studied with anything like completeness 

 — all at least above the Protozoa — show that the germinal 

 matter which results from the segmentation of the ovum 

 differentiates two concentric germ-lamellae out of which the 

 whole animal body is built up. In animals above the 

 Ccelenterata there is further formed between the outer and 

 inner germ-lamellae a middle germ-lamella, but whether 

 derived from the outer or the inner remains an unsettled 

 point. This middle lamella is not formed in the Hydroida 

 unless, as Kleinenberg with considerable reason supposes, it 

 be represented by the fibrillated layer or muscular lamella, 

 and as there can be no doubt that this really belongs to the 

 ectoderm, an argument is afforded in favour of the middle 

 lamella being a derivation from the outer. The muscular 

 lamella, however, is not in the Hydroida an independent 

 layer ; but, as Kleinenberg has shown, passes continuously 

 into the siiperficial cells of the ectoderm. The ectoderm 

 should thus be regarded as the united outer and middle 

 germ-lamellae of the higher animals. 



It should be noted that the study of hydroid development 

 proves that the digestive cavity is here formed by a simple 

 hollowing out of the interior of a solid germ, the mouth 

 subsequently making its appearance by a rupture of the 



