DEVELOPMENT OF TEE FORMS OF ANIMAL LIFE. 



4S7 



on and fertilized, then proceeds to undergo the 

 changes of development by which the foundation 

 is laid for the new plant. 



In the alga? and fungi, however, there are 

 gradations of the differentiation of the two re- 

 productive cells, which are of the greatest inter- 

 est in leading to a comprehension of the general 

 nature of the formative process. For in the 

 lower and simpler forms of these plants, such as 

 the Desmidise, Mesocarpea3, and other Conjugatee, 

 we find that there is no distinction in structure 

 or form to be perceived between the two cells 

 which unite or undergo conjugation ; and a com- 

 plete fusion or intermixture of the two masses of 

 protoplasm results in the production of a single, 

 usually spherical, mass holding the place of an 

 embryo. And that there is an absence of special- 

 ization between the two uniting cells is clearly 

 shown in both Desmidium and Mesocarpus, by the 

 fact that the embryo or zygospore is formed in 

 the mass resulting from the union of the pro- 

 truded portions of the two cells ; and in more 

 ordinary cases, as in Spirogyra, where the em- 

 bryo is formed in one of the two cells, it seems 

 to be indifferent in which of them it is formed. 



From this, which may be regarded as the 

 most elementary type of new production by the 

 union of the two cells, the transition is not a 

 great one to the development of a progeny with- 

 out any such union. We might conjecture, then, 

 that the capacity for separate or individual exist- 

 ence extends in the lowest organisms to the 

 whole, or to each structural element of their or- 

 ganization, while, as we r'.se in the scale of vege- 

 table life (and the same view might apply to the 

 animal kingdom), this capacity is more and more 

 divided between the two productive elements, or, 

 at least, is only called into full action by their 

 combination. 



The germinal element consists of a simple 

 primordial cell, varying in different kinds of 

 plants, but in all of them probably containing the 

 essential substance protoplasm, and the most im- 

 mediate result or effect of fertilization is the mul- 

 tiplication by repeated fissiparous division of the 

 previously-existing cells. The new individual re- 

 sulting from this cellular growth usually remains 

 within the parent body, without, however, direct 

 union or continuity of tissue, till the embryo has 

 attained some advancement, as in the well-known 

 case of the seeds of a phanerogam ; but there are 

 many varieties in the mode of its disposal among 

 the lower plants. 



A remarkable exception to the more direct 

 relation of the process of fertilization to the for- 



mation of the new individual or embryo occurs 

 in some plants, simulating in some respects that 

 kind of variation in animal reproduction which 

 has been named alternate generation. A well- 

 known instance of this belongs to the vascular 

 cryptogams. The prothallium of the ferns, for 

 example, results from the development of so-called 

 spores or unicellular buds, which are familiar as 

 being formed in small capsules on the lower leaf- 

 surface ; and in this prothallium, when it has 

 reached a certain stage of vegetation, there are 

 formed the archegonia, containing the oospheres, 

 or germ-cells, which are fertilized by the moving 

 ciliated particles developed in the cells of the 

 antheridia, leading to the production of a new 

 spore-bearing plant. 



Recent researches have also called attention 

 to the remarkable arrangements in phanerogamic 

 plants for the prevention of fertilization of the 

 pistils by pollen from the same flower, or even 

 from the same plant. In the latter case this is 

 effected by the separation of stamens and pistils 

 in different flowers on the same or on different 

 plants. In the former case, where both organs 

 occur in the same flower, the adaptations, whether 

 of a mechanical or of a physiological character, 

 by which self-fertilization is prevented, as ascer- 

 tained by numerous recent investigations (among 

 which those of Darwin are most conspicuous), are 

 of the most varied and often the most compli- 

 cated kind. 



Let us now turn to the consideration of the 

 development of animals, and let me say in the 

 outset that it will be necessary for me to confine 

 my remarks chiefly to the higher or vertebrated 

 animals, and to certain parts only of the histo- 

 ry of their development ; more particularly the 

 structure and formation of the ovum or egg, its 

 earlier developmental changes, and the relation 

 of these to the formation of the new animal. 



I cannot enter upon the consideration of this 

 topic without adverting to the very recent ac- 

 quisition of some of the most important facts 

 upon which this branch of knowledge is founded ; 

 and I feel it to be peculiarly appropriate, in the 

 year of his death, to refer to a biologist whose 

 labors contributed more powerfully than those 

 of any other person to give to animal embryology 

 the character of a systematic branch of science, 

 and to whom we owe some most important orig- 

 inal discoveries — I mean Karl Ernst von Baer, 

 of Konigsberg, St. Petersburg, and Dorpat. 



Of observers who, previous to Yon Baer, were 

 mainly instrumental in preparing the way for the 

 creation of a more exact modern science of em- 



