328 



Embryogenesis: Progressive Differentiation 



too large (Fig. 124A), often covering as 

 much as three-fourths of the surface, al- 

 though it later disappears and the surface 

 becomes uniformly ciliated (Fig. 124/1'); 

 (2) the absence of both stomodaeum and 

 ciliated band, normally derived from cells 

 of the animal half; (3) the absence of skele- 

 ton; (4) failure to gastrulate. Such larvae 

 are said to be "animalized" or "ectoderm- 

 ized." But in less extreme cases the apical 

 tuft is more restricted (Figs. 124C, D) and in 

 such cases a ciliated band (Fig. 124C') and 

 even a stomodaeum (Fig. 124D') may de- 

 velop (Horstadius, '39). The vegetal halves 

 of eggs, the isolated animal halves of which 

 produce overdevelopment of the apical tuft, 

 may form quite normal plutei (Figs. 124£, 

 F), or they may form exogastrulae (Fig. 

 124G) with poorly developed skeleton and 

 arms. At the other extreme the vegetal 

 halves of eggs, the isolated animal halves 

 of which produce a restricted apical tuft 

 (Figs. 124C', D), may produce very abnormal 

 larvae (Figs, 124H,I,J). The latter consist 

 mostly of gut contained within an ecto- 

 dermal vesicle which may develop a ciliated 

 field at its most animal border (Fig. 124//) 

 and a skeleton which has not advanced be- 

 yond the spicule stage (Figs. 124//, /) or 

 which may be entirely absent (Fig. 1247). 

 Such larvae are said to be "vegetalized" or 

 "entodermized." 



It is not necessary to separate the animal 

 half completely from the vegetal half in 

 order to produce such developmental modi- 

 fications. If the egg is simply stretched along 

 its animal-vegetal axis (Lindahl, '36), or 

 if a constriction is formed by a single silk 

 fiber around the equator of the egg (Hor- 

 stadius, '38), the partially separated animal 

 and vegetal halves develop much as if they 

 were completely isolated. Thus it appears 

 that some influence from the vegetal half 

 (vegetalizing influence) normally passes into 

 the animal half and exerts the following 

 effects upon development of the latter: (1) 

 restriction of the apical tuft; (2) stimula- 

 tion of stomodaeum invagination; (3) stimu- 

 lation of formation of a ciliated band on 

 the ventral side parallel to the animal-vege- 

 tal axis. Similarly, it appears that some in- 

 fluence from the animal half (animalizing 

 influence) normally passes into the vegetal 

 half and exerts the following effects upon the 

 latter: (1) restriction of the size of the gut; 

 (2) control of skeletal development. Recent 

 experiments (Horstadius, Lorch and Dani- 

 elli, '53) seem to demonstrate that enucleated 

 vegetal halves cannot exert vegetalizing in- 



fluences, but that enucleated animal halves 

 can still exert animalizing influences. 



Time Required for Animalizing and Vege- 

 talizing Influences to Exert Their Eftects. Ex- 

 periments designed to answer this question 

 can best be summarized by paraphrasing the 

 words of Horstadius ('39, pp. 160-ibl). 

 Animal halves are isolated every second hour 

 from the 16-cell stage (4 hours post-fertiliza- 

 tion) up to the beginning of gastrulation (16 

 hours post-fertilization); if isolated 4 to 6 

 hom's after fertilization they produce a 

 large apical tuft and develop into ciliated 

 blastulae or blastulae with a ciliated field 

 (Figs. 124 A,A',B,B'), whereas if isolated 

 later (10 to 16 hours post-fertilization) they 

 produce a restricted apical tuft and develop 

 a ciliated band and stomodaeum (Figs. 124D, 

 D^), the structures which they should pro- 

 duce as parts of the intact egg. The change in 

 differentiation of the animal halves occur- 

 ring about 8 or 10 hours post-fertilization 

 (blastula before formation of primary mesen- 

 chyme) indicates that by that stage the 

 vegetalizing influences have so interacted 

 with the animal half that the latter can then 

 self-differentiate according to its prospective 

 fate. Isolation of the most animal quarter of 

 the egg, corresponding to an^^, indicates 

 that vegetalizing influences have modified 

 its fate somewhat later, from 14 to 16 hours 

 after fertilization. This indicates that de- 

 termination, as regards the influence of 

 vegetative material upon the animal, pro- 

 ceeds from the vegetative towards the animal 

 pole. The isolated vegetal halves produce 

 ovoid plutei until 14 to 16 hours post-fertili- 

 zation when suddenly they develop into a 

 new type of larva, a pluteus with long anal 

 arms, but entirely without oral lobe, i.e., 

 they develop as if still part of the entire egg. 

 This change in differentiation indicates that 

 by that stage the animalizing influences have 

 so interacted with the vegetal half that 

 the latter can self-differentiate according to 

 its prospective fate. 



Do Isolated Animal Halves Lose Their Com- 

 petence to Respond to the Vegetalizing Influ- 

 ences at the Same Time that Capacity for 

 Self-diflFerentiation Is Attained? To answer 

 this question animal halves are isolated from 

 vegetal halves at progressively later stages 

 and micromeres are then added to such iso- 

 lated halves to determine how late a nor- 

 mal pluteus with gut can be induced from 

 animal halves. If four micromeres are added 

 to isolated animal halves even as late as the 

 stage when onset of gastrulation normally 

 occurs (16 hours post-fertilization), a gut 



