120 COMPARATIVE ANATOMY CHAP. 



nourishment of the developing animal is a secondary addition to the original develop- 

 ment. We cannot think either of a bird embryo at the time of incubation, or of a 

 mammalian embryo in its egg envelopes as an independent and self- feeding animal. 



4. Direct development may occur together with gradual development or meta- 

 morphosis in the ontogenetic development of the animal. Of all the original in- 

 dependent stages of development, often only one or a few are preserved, viz. those 

 which are specially capable of competition, whilst all the others disappear in the 

 direct development. Thus the insect egg develops direct into the caterpillar or 

 larva by the help of the nutritive yolk. The larva at this stage of development, 

 being suited for competition, procures food for itself independently, grows vigorously, 

 and then again develops direct (pupal stage) by the help of the stored -up food into 

 the adult winged insect. 



5. The effect of the struggle for existence on the larvse at the various stages of 

 development, and especially on those which feed independently, is just the same as 

 on adult animals. It is therefore to be expected that the different manners of life 

 (attached, free-swimming, parasitic, etc.) during the stages of development should 

 determine adaptations and modifications in the larval organisation similar to those 

 in adult animals. Such modifications, however, have a limit not clearly known to 

 us, just because the stages they affect are stages of development whose purpose is 

 to produce an adult sexually mature animal. 



6. The more important organs or systems of organs are, the earlier do they 

 begin to form in the larva ; or, in other words, the order of their development in 

 time is in direct correspondence with their importance to the adult animal. 

 Whatever holds good for the adult animal also naturally holds good for every 

 organ and for every adaptation ; its development becomes more and more direct, and 

 more and more abbreviated ; it adapts itself more and more to the purpose of reaching 

 as soon as possible the form and arrangement which belong to the adult animal. 

 Increasingly defined localisation of the developing parts is a necessary consequence 

 of their earlier commencement. 



We see from the above that in phylogenetic investigations it is perhaps still 

 more difficult to decide the true bearing of ontogenetic than of anatomical facts. 

 We can only attain to phylogenetic conclusions of a certain degree of probability 

 when comparative anatomy and comparative ontogeny go hand in hand, when com- 

 parative anatomy takes into account the developing organs, and when comparative 

 ontogeny does not leave out of consideration the last stages of development. 



Segmentation and gastrulation. When we come to investigate in what manner 

 the bi-laminar germ, the gastrula of the Metazoon, arises out of the fertilised egg, 

 we find what appear to be very different methods of development. Numerous 

 thorough investigations have proved that this variety of methods is almost exclu- 

 sively caused by the quantity and distribution of the nutritive yolk in the egg. 

 If we assign to the influence of the nutritive yolk the importance that is due to it, 

 we shall be convinced that one single process underlies all these different phenomena. 

 We must keep well in view, (1) that the nutritive yolk, or deutoplasm, is an 

 inert, lifeless nutritive material deposited in the egg cell, and (2) that the forma- 

 tive yolk, or the protoplasm, with the nucleus it encloses, is the only living active 

 portion. 



We have already spoken of the variations in amount and distribution of the 

 deutoplasm in the egg. We shall now describe the first appearance of segmentation 

 or furrowing in the different kinds of eggs. The following are the types of eggs 

 whose segmentation we propose to describe : 



Holoblastic alecithal egg. Holoblastic telolecithal eggs, with varying quantity 

 of nutritive yolk : Eupomatus (Annelid), Discocelis (Polydad), Bonellia (Ecliiurid], 



